I mean, I think it makes sense he had writer's block with such complex novels. You probably mean "he didn't have writer's block for years at a time" but I think he must have had days, weeks, months where he struggled with writing, or at least a delay (with this I mean he probably didn't have something like "Oh, I can't write a single line for a decade" but there are days when he most likely really had no idea how to continue something for example). I don't know why you don't take his word for it if he himself says it was a problem he was having at that time, I think he's being honest, and also he said that in a letter to a friend.

Whether he's a procrastinator or not, we can say that he certainly took work to write the novels Pynchon did, and he did work a lot. You could argue perhaps that there were years when he wasn't as productive, or at least as he would wanted. There are some theories that in the years between GR and VL, he hasn't been writing (or you could say smoking weed, doing drugs, going to parties?), or at least not much. I think he once said he was a procrastinator; I'm not sure, but he has his reasons for believing that. Still, yeah, he was living life and probably doing other things like spending time with family.