Well I wonder about that. While the multiplayer in Tony Hawk's HD was only 4 players on consoles, it featured some of the same modes already (and others like Graffiti which apparently isn't in THPS5) so it's debatable how much time it actually took to adjust it for THPS5.
Probably still long, but some people are putting it like Tony Hawk's HD didn't have multiplayer at all so that's where all their effort went. But no MP is only true for the PC version.
I only played the PC version so that is actually what I had thought. Thanks for clarifying.