That conclusion certainly isn't supported by the data or empirical evidence.
I assume that your per 40 calculations are adjusted for minutes played. Jesse is a plus defender who blocks ~3bpg, and 4 per 40. Frank blocks 1 per 40. Jesse is the only defender in our primary rotation who appears to be elite, and his ability to block shots and alter ones he doesn't get helps to compensate for the limitations of his teammates.
And blocks certainly don't tell the full story with respect to defense. We just saw in the last game what happens when Jesse isn't playing, and Frank is. BC attacked the rim with impunity, and did a ton of damage down low. That doesn't happen against Jesse.
Now, it is possible that playing more will enable Frank to get more blocks, but again -- if your data is already adjusted, then the difference there between the two is stark.
I can get behind the notion that they are comparable on the glass. I'd also expect Frank to elevate his scoring a bit, with more PT and the ability to not to worry about getting pulled should be good for his confidence out there.