I think the difference is most Jam members actually make their game from scratch, including the basic idea, in the 72-hour time limit.
If people can manage to write interesting development logs during Ludum Dare (a 48 hour comp, so even more time limited), then surely people could put in some effort here.
On Ludum Dare, I often feel that many (really, a lot!) of games are mostly complete beforehand. It's hard to believe authors can spend large amounts of time blogging during that 48-hr window... and then release a very polished looking game.
Not saying all LD participants do this. But I believe a very large number do.
I don't think that's true in the slightest. You can see the proof by watching the myriad time lapse vids, or even just by participating in one (you'll see what I mean).
I've participated in LD and made everything from scratch several times, and ended up with a decent game after 48 hours, while also doing extensive development logs (more in depth than the ones I've done for GMC jam )
So, I'm confident you'd find the majority of LD entrants do actually work within the time constraints. Given there are no prizes whatsoever, the fun itself is the challenge and I think most entrants to LD realize that.
For Ludum Dare you have to make everything from scratch (including music, etc.) and there are only 48 hours, and yet the process is still far more engaging from a community viewpoint than the static posts I've seen during the GMC jam.
And for anyone that never has done Ludum Dare, I'd strongly recommend it. It's an amazing experience