I guess the bottom line is that I don't know all the physics that go with the solar system, so sometimes I don't know whats-what in planetary works.
I did have a college professor that said it's people like me that end up making theories on what happens if we blew up the moon. LOL.
At the time of course, I was like WTF. Of course when we talked about plate tectonics I had to ask if it could of be caused by the same protoplanet that is suppose to have hit the Earth to create the moon which could of cracked the planets mantle.
I was told to stop asking questions. LOL. <(O.o)>
Asking what may appear to be a silly question is only the first step.
How would you go about determining whether or not plate tectonics could have been caused by a large collision?
What assumptions have to be made for that to be valid? What evidence can be found that correlates with those assumptions? What behavior could be predicted, then verified?
It isn't enough to just ask a question; the follow through is important. And frankly, classrooms aren't the best place to go into that level of detail every time someone asks similar questions. Properly answering questions often requires a good deal of effort, knowledge, and experience.
But to answer your question regarding tectonic plates, the likelyhood of that being the cause is extremely low, as tectonic activity is present on other planets/moons which have no evidence of undergoing similar scale impacts (above and beyond any other reasons).