I wonder if Grab is liable in any way if an accident occurs.

Yes.  But if want to sue them, are they liable? 

While the drivers, delivery people are not under their employment, they do use their platform.......

I am no lawyer but if you get into an accident with a vehicle that belongs to a company, you can sue the company too. 

See - https://asklegal.my/p/claim--traffic-accide...mages-insurance
under 'You can get the driver’s boss to pay up'

Question is whether can sue other companies such as Grab.  They don't own the vehicle but the drivers are out and about providing a service using their platform.