Back then it had been all fun and games, and Sirius had admittedly gone a little out of hand with that whole incident. Especially now, in retrospect, it was awful. Because he didn't want to be the reason for anyone's death. And moreso, he didn't want Remus to be made that reason either. The guilt would have buried them both, and it would have been all Sirius' fault. He did have regret for that. They had moved past it. If only because Remus was a loving and benevolent sort.
The Doors made absolutely no sense at all, but frankly they were both rooted to worlds of magic - time travel was not so out of the question, so why would traveling through different spaces be so different? Sure, he couldn't quite get his head around the specifics, but he could say it worked. And sometimes? That was enough for Sirius. At least, for now. Later, maybe, he'd try dissecting it into something that could be sorted.
The problem with the Door situation was that whatever was behind those opened doors was forever changing. They could find something sufficient this time, perhaps, but Sirius wasn't completely positive the odds would always be with him. They could hope, of course. "Yeah," he said, clearly still mulling it over. "That might be best. Maybe -- I don't know. Maybe it would be smart, y'know. To pull one of those blokes who are practically mayor aside and tell them the skinny of it?" It'd be like having back up, but it wasn't an open letter that'd invite everyone into their business.