My understanding is that Fati would not have been offside because he was behind the ball IF Welbs had passed it. Am I right or am I wrong?
That's (no offence) obviously correct. The (incorrect) suggestion was that Fati was offside when the previous ball was played to Welbs, and as it was the same phase, he'd then be offside had Welbs have passed it to him. It's only whether or not you're in an offside position when the last player on your side to touch it, passes it to you.