You are correct that it should be motor_vehicle:conditional instead, so I will correct that. Changing what was there to maxweight=3.5 is incorrect though, and I did revert that, but wanted to know if you had any local knowledge that I might've missed in my mapping of the access restriction.

For an explanation of how it's signed:
* No motor vehicles
* (over) 3.5 tonnes
* Does not apply to public transit buses

Hence a regular weight restriction is not the right way to go.