Yeah, Taiko does it the TnT way, which is to have the velocity of the notes constant. Different notes can have different speeds, but each note will keep the speed it starts with for the duration of the time its on the track, meaning that the space between two notes moving at the same speed that appears to be 1/2 beat will still be 1/2 beat when it gets to the drum. But this means that notes moving at different speed have the distance between them variable, which results in the appearance of different "conveyor belts", where a third note can suddenly race in and get in between those two 1/2 beat apart notes.
The proposed way for mania flips things around (switching the time domain for space). The thing that's constant is the relative distance between notes... so that there can only appear to be one conveyor belt because everything is firmly set in place. However, the speed of the belt can change and so the notes now have variable speed and a distance that appears to be, say, 1/2 beat, might not be 1/2 beat when it gets to the bar because all the notes change to the same speed when the speed changes.
The reason for including SV even though BPM could do the job should be obvious. BPM shouldn't represent anything different than the actual BPM of the music, otherwise there will be problems with all the different modes that might be trying to read the map. SV is the appropriate value to use for signaling a non-BPM speed change.