Never mind, I just saw the comment section of MoBamba4 youtube video and his run is not fully 100% yet so he is put in Any%.
They only completed 50 instead of the required 100 Taxi Fares.
They could have easily confirmed that whether they have 100%, by loading the save made after the last mission and checking the Stats menu, but they didn't. Had they checked, the game would have shown 99%.
At 7:18:20 of part 2, before starting the story finale, they checked the Stats menu but didn't scroll down all the way to the side missions.
At 4:02:10 of part 2, they delivered their 50th Fare. However, as you can see the mission continued instead of ending and MoBamba had to cancel it manually. Unlike other games in the series, GTA Advance doesn't allow the player to continue or restart finished side missions. Had he (re)entered a Taxi afterwards, the game would have displayed the prompt about starting Taxi Driver, indicating that the vehicle mission is not fully completed yet.
All of this makes it obvious that the cause of missing the objective was the lack of knowledge. While GTA Advance documentation is (/ used to be) hard to come by, both GrandTheftWiki and GTA Wiki (Fandom) had the correct information about this objective for years prior on both the Completion and Taxi Driver pages. Unfortunately not enough research had been put into the category before attempting their run.
Ultimately, Ethan had to decide whether to decline the run altogether (probably making the run lost in the long run) or moving it under Any%. Arguably, 99% fits Any% better than 100%, because the last % is not unobtainable in this game.
You can find the list of required objectives and explanation here: https://www.speedrun.com/gtaa/thread/ew2n8/1#oy9fc
Hopefully this wall of text will clear the confusion once and for all.
