Maybe this could be considered another historical event/marker of the start of the European Renaissance alongside 1492 and 1453.
It might've helped that Barbarossa was previously a Christian pirate before becoming a Muslim admiral in the Ottoman navy. A sign he's not as motivated by religious dogma than by power. It's much easier to trust someone when interests align.
BTW, there's a new Turkish TV series based on him (the same lead actor that played Ertugrul in that smash hit series)
You are right the was always Muslim.
In Islam, it is common belief that all people are born Muslim, then their parents may choose to lead them away into another religion.
That's why for people who convert to Islam later in life (muallaf) it is sometimes said that they reverted.
> Every child is born with the fitrah, but his parents make him a Jew, Christian or a Zoroastrian. ~Al-Bukhari
I guess that makes him a bit of a Christian too but nowhere as close as being a Christian pirate who switched sides.
That said, it was a figure of speech. He was definitely not a Cristian converted to Islam and whoever thinks otherwise perhaps got confused because of the notion that his mother was a Cristian perhaps?