What occurred is possible. Based on my source, I'm guessing that you were quite far in the tower.
As you know, Leon uses Dynamax on a Pokémon on his team, being Charizard. However, there are some other trainers that do so too, on the Maximum Rank of the Battle Tower, including:
- League Staff Lazlo
- League Staff Allegra
- Youngster Ander
- Lass Hestia
- Poke Kid Alan
- Poke Kid Eva
- Police Officer Cooper
- Beauty Becka
As stated, all of these Pokémon Trainers can Dynamax their Pokémon, and can be found at the Max Rank of the Tower.
Source
