In principle it should be trust in the experts as scientists from Institute of Medieval Research, Vienna, a department of Austrian Academy of Science. People should read whole text of Dr Borri and they would understand about narrative and legendary elements in emperor’s story and his motives for alliance Byzantine with Croats against Bulgarians.
First we should distinguish what in DAI is a "emperor’s story", and what is not. Historians mostly agree that the chapter 30. does not belong to the original Constantine's work and it is probably added to it after his death. If the text was not written by Constantine, than how can we talk about "emperor's motives"?
Nobody mentions Croats in Dalmatia and wider before 9th or 10th century.
They were mentioned under different names as it was clearly stated in Historia Salonitana:
"The people called Croats...Many call them Goths, and likewise Slavs..."
When they appeared in Dalmatia, Croats didn’t wear T-Shirts with their etnonyme on it so everybody could know how they called themselves. In the beginning of the 9th century it wasn’t possible for them to write down their name in
their own language. How could any foreigner know what a hell the word “Hrvat” (Croat) mean? What do you think how many people know even in 20th century how Finns call themselves?
Dr Borri put emergence of Croatian identity in 10th century but Croatian historians think that document where is mentioned Dux Croatorum from 9th century should be valued as first document with Croatian name. Croatian historian Vidovic writes that Croatian identity was present in the middle of 9th century. Therefore question between historians about emergence of Croats is if 9th or 10th century, not before.
Here is the duke's Branimir inscription dated to 888 AD: "DVX CRVATORṼ COGIT[AVIT]"
https://en.wikipedia.org/wiki/Branimir_Inscription This is also worth noting:
possibly older reference to a Croatian ethnonym is attributed to a charter of duke Trpimir I of Croatia from the year 852. However, its original is lost, and only a 1568 copy is preserved, leading to doubts on the authenticity of the claim
But Croatian history did not start in the year 888. Before Branimir there were other dukes. We can easily extrapolate Croatian presence back to the beginning of the 9th Century.
History is known, don’t listen nationalists who like to confuse. When Slavs arrived in the Balkans from 6th century chronicles of that time were marked them as Sclaveni (Greek Sklavenoi) and Antes. National identities emerged later.
During the first decade of the century it was fashionable for historians to deny the migration. Influence of the early Y-DNA studies

? Possible. Now it changes again and we expect new generation of historians to give us more plausible explanations. Btw. you noticed well that the ethnonyme Sklavenoi was invented by Greeks.
Croatian ethnonym is not Slavic what Croatian scientists highlight, not only non-Croatian. There are more hypothesis if it is Avarian, Iranian, Bulgarian, Turkish, Scythian etc. I appreciate Croats and don’t want enter in speculative discussions, it question is not solved.
Are we talking about the origin of the word or the origin of the people? Croatian ethnonym most probably has a root in Iranian languages, but this is far from being the only Iranian loanword in early Slavic. Consider another word of Iranian origin - “bog” (god), which is present in all Slavic languages and represents one of the most basic terms of any society. Btw, as a Serb, don’t you agree that religion sometimes plays a key role in forming of nations?
But in Slavic word it is almost normal that Slavic nations have non-Slavic ethnonymes. For example Bulgarian Slavs took name which is not Slavic. Slavic Macedonians too. And these days in Skopje we can hear about "White Macedonians". Why people create such constructs? To give greater importance to their nation? It is wrong approach but nationalists don’t care if someone wants to follow them.
Bulgarian case is not same as Croatian. Slavs in Bulgaria got their name after the country they lived in. Slavs outside Bulgaria never identified themselves as Bulgarians. On the other hand, Croatian ethnonyme was recorded in almost all areas where Slavs migrated: Poland, Ukraine, Czechia, Lusatia, Slovenia, Austria, Montenegro, Greece, Macedonia, Belarus, … the name they obviously brought with them from their homeland. I never heard about “White Macedonians” theory. What does it have to do with Croatian history. I don’t get the point.
Really only Ukrainians, Poles, Slovaks and Slovenians have undeniable Slavic ethnonym. Even for Czech people is not sure is it Slavic or no. And some small Slavic nations as Kashubians seems have no Slavic ethnonym.
Except maybe Poles and Czechs, what do you think when those nations got their present names?