Here is the sign according to Streetview:
View attachment 552303
I guess it's possible that GO had 'authorized' other companies to use the lanes.
It's not a matter of being red or green, it's a matter of being trained to use those specific bus lanes.
Only specifically authorized vehicles are permitted because that the lanes are severely substandard and therefore have additional rules for operation. If I recall correctly from my TTC training, you can only drive in those bus lanes when traffic is moving slowly, and you are not allowed to exceed 80 (?) in the lanes.
If they just allowed any random charter bus in the lanes, you'd end up with some out-of-town bus driver trying to go full speed in the lane and crashing.