[smt88]

Security doesn't always mean "seeing something you're not supposed to see". He's saying that the boards are public, so people are able to just change the number at the end of the URL to find them all.

You can have the same issue with scrapers. It's much easier for scrapers to get all your pages if you use sequential numbers for unique IDs.

Yes, a search engine could index the pages, but the big engines will obey your robots.txt, and the small engines will never know that you exist most likely.

So s/he's not trying to "secure" anything as much as just hide it.