Not OP but overexplaining jokes is my PaSsIoN.
If you do a raw substitution of Roman numerals for Arabic numerals when writing FF15, you wind up with FFIV. Of course, because of the way Roman numerals work, FFIV ≠ FF15 (as that is properly written as FFXV). Instead, FFIV refers to Final Fantasy 4.
The joke is that the only way you can make FF15 (or FFXV, if you prefer) good is to misunderstand the numbering and play a different game instead.
Edit: aw beans someone beat me to it.

















Feels like that could be a minor villain from Captain America’s Silver Age rogues gallery.