Its not a main battle tank. Its a personell carrier with a turret. The armor is designed to stop/deflect small arms fire.
And yes, a hard bottom of known depth is still going to be required. Back pressure on the valves would be an issue, as would water intrusion into all of the bearings.
Another problem is buoyancy. I’m surprised that this thin skinned vehicle isn’t positively buoyant. Even at that, it has to be very light on the treads when full submerged.
This type of snorkelling is nothing new, but has never really caught on. There are too many things that can go badly wrong.
The Russians did this during the Cold War. Their poorly trained conscript troops would sometimes make mistakes, like deviating from the surveyed (and sometimes even prepared) path across the river. Accidents, like bogging down in the mud, getting pointed downstream by the current, hitting a hole, or vehicle failure happened with surprising frequency. Sometimes, they lucked out and an extraction vehicle could pull them out, and sometimes they died on the bottom of the river.