I honestly have no clue what "the d20 engine" even means in this context.

Like I can say "Pendragon uses the d20 engine" and be serious because it's throwing a d20 +/- stuff vs a target number, and it was already on it's 4th edition in 1995 before TSR collapsed. There was a Diablo d20 table top that was hilariously bad and would have been complete **** if ported back to the computer with it's rules. You can take freaking Gurps or Champions into using a d20 with just some basic scaling math conversions while keeping all the actual % chances and bell curves the same and call that "using the d20 engine".

So yeah, no clue. The actual dicing for probabilities and character building bits are functionally trivial compared to deciding what actions & interactions you're going to encode for the computer.