Why does Epitaph of King Ferdinand in Capilla Real, Granada, Spain say he was the servant of Allah?

In Arabic there is no other way to say "God" apart from simply "Al'lah," so Arab-speaking Christians even in the modern day refer to God using this word but meant distinctly from the God of Islam. So in that sense, yes, one could say that Jesus = Al'lah.

I once saw a Malay translation of the Bible in which the word "God" was translated "Al'lah" throughout, even though there are other words for God in Malay. There's some influence here, it would seem, since Malaysia is a Muslim state, and so most Malaysians would understand the word to mean simply "God."

In Swahili-speaking countries along the East African coast, it is not uncommon to hear people say "Insh'Al'lah," meaning simply "Godwilling," regardless of their religious persuasion. (Swahili is an East African language with Arabic roots.)

So Ferdinand's epitaph is just an Arabic translation of "Servant of God." The show you watched might have put it in a misleading way.

I think it was just one of those things he felt like doing at the time.