Muslims, how do you react to this experience of mine with your fellows?

With so many Muslims I've had debates with regarding history this has been a phenomenon: Complete and utter denial of the history of the religion and lies about what has transpired. Only recently a Muslim I conversed with on a historical channel regarding slavery in Islam vehemently claimed that no slavery ever existed, despite us having abundant documentation of it being so throughout the four caliphates, and during the Ottoman Empire who engaged in the African Slave Trade and abducted Christian youths from the Balkans for generations to be trained as Janissaries. Enslavement of fellow Muslims was indeed forbidden, but not of pagans and Christians.

This man constantly demanded evidence and asked inane questions like "what were the slaves' names then", (obviously we neither know nor need to know that to prove it), but when presented with several references straight up claimed that all western sources as soon as speaking negatively on Islam were biased and false, "western sources lie about Arabs and all other peoples". Even so we know of these facts from eastern sources as well, and he failed to be able to specify what made a "western source" different from whatever source he accepted, which seemed to be only those who claimed it didn't happen, which doesn't exist and neither did he ever offer any counter source. He also conveniently forgot about Mauritania, an Islamic country with the highest level of modern slavery still in existence.

I'm just honestly tired of this BS attitude from many within Islam, this trend of denying the obvious history of self-aggrandizement in Islam. It's like they would have us believe that Muslims and Islam both have been flawless throughout the ages and the best thing to ever grace the earth, that not a single human error has ever been committed in its name. I have encountered this enough times now to feel like it is a rather common approach Muslims have, but I would love to be proven otherwise.
Muslims, how do you react to this experience of mine with your fellows?
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