It is possible to swap NPC models around -- and the models shown in this video all exist in SRIV, which can be ported back to SRTT with the right tools and such. The player model is more complex, since it doesn't exist as a single mesh or asset, and it's not part of any of the game's external data files (it's likely embedded in the game's executable). But you *can* get around this by using costumes which can partly or fully replace the standard character model with another one -- like the Iron Saint costume in SRIV. As long as they're rigged the same way as the player model, that is. And somewhere in the SRTT's files are the 'outfit' definitions, including the ones that determine what the player starts out wearing. And that's what it looks like is being done here. Someone converted the Julius Little character model from SRIV into a costume, ported it back to SRTT, and set it up as one of the default player outfits.
It's technically complex, and I wouldn't recommend trying it for anyone not intimately familiar with all the ins and outs of SRTT's files structures and data streaming system.