Where do tar pits come from? The bitumen came from somewhere. Does it need to come from plants?
Tar pits are just another form of hydrocarbon seep.
Does it need to come from plants?
No. This deep Earth theory is one alternative. However, to my knowledge, no geologist has been able to use this deep Earth theory to predict where oil might be found whereas, using classical "squeezed dinosaur" theory, geologists can predict where to look for oil.