To be honest, I didn't think about it too hard - I just wrote "a XCF" because it doesn't start with a vowel. I don't think it's worth submitting an edit for, but you're welcome to consider it "an XCF".

A "ksceef"?

But yeah, in my mind, it's always been "ex see eff", so an XCF.

I find some word processors don't test for vowel sound, and only adhere to the actual vowel letter.

In my mind it’s similar to exif

The beginning of “XCF” sounds similar to the beginning of “Exit”.

With “a” vs “an”, the pronunciation is more important than the spelling.