There's a big difference between bulk and strength.
Bulk means your harder to push around (obviously, because your heavier) so he put on 40 pounds he said over the 2 years of college.
Strength normally refers to core strength, how strong your core is (most important in this case).
Some guys are don't look very big, but have ridiculous core strength (MKG is the best example I can think of, he's not big at all but gee he really is strong in the core).
Others are big and as a result of their bulk can push people around (Z.BO is a good example, Shaq to some extent since he was so large, but he was really strong as well).
The ultimate specimen is someone like Lebron James or Dwight Howard. Both of them have ridiculous core strength and are huge as well to reflect that.
What Len has done is bulked up to 255, which is excellent. He spoke of 265 being his ideal playing weight. And since he was on a budget, he put on weight via peanut butter and jelly sandwiches, aka fat. Not muscle, but fat. A lot of gymmers will do that to become big, if you're gaining weight, then it's good. Same reflects Len. He NEEDED to get bigger, so he went high calorie foods. Nothing wrong with that, since his aim was to get heavier so he wasn't pushed around in the post so much.
Sure he was in the gym, but a lot of his calories he took in was in the form of high calorie foods, and fat doesn't help you. Sure you're slightly heavier, but he's not like Biyombo who is just about all muscle (he's got excellent core strength, but doesn't look that big either).
When we think of Biyombo, he's a chiseled 245 pounds, with less than 10% body fat. Taking that into account, Biyombo only has at maximum 24 pounds of fat, while the rest of his 221 pounds is muscle+bones. While in comparison Len is 255, but of that 255 probably 30+ pounds is in fat (haven't seen his fat %, but I expect it to be at minimum 35 pounds or so)
And that will help explain his lack of strength. He put on pounds yes. But he needs to hit the gym HARD in order to build his strength up to bang in the post.
It takes a long time to build up strength and a ton of hours in the gym with a lot of core work and heavy lifting.
Luckily for him during rehab, he has the time to work on his body.