Just because champion.gg favors the build with more gameplay doesn't change that it was (and probably still is) the highest winrate build.
That's not how champion.gg works. The Liandry's build had over 400 wins. It didn't get kicked out of the highest winrate spot for not having enough gameplay. It got kicked out for winning less. It's not the highest winrate build anymore. It's not terrible, but it's not the highest winrate option.
Idk I played support for a MF last week who maxed it and built AP. She came back to lane with 1 itam at 6 and double killed their bot lane with just ER.
And I've supported for an AD karthus who maxed E about 6 months ago. We went 9/1 in lane. Just having one game where it works doesn't mean much. E maxing is easier to play against than Q maxing if you know how to play against it. I've played against MFs who have maxed E before, and they're much more vulnerable to being hit with an all-in or just to having their harassment dodged. It's not like it's incredibly bad, but it is pretty much always worse than maxing Q.
It should also be noted that part of why this worked for your MF was that she was building AP. That makes E maxing significantly better. AP MF can get away with it because E will do enough damage to clear waves. AD MF doesn't even have that advantage.