They usually have some good intel when they start which gives them credibility. Most of their sources though are from marketing agencies, merchandising, recruitment, retailers, QA contractors and (third party) developers.
Sometimes people simply share too much on their linked-in profiles or portfolio's.
Obviously no one can have sources within every level or studio and seldom do folks actually have a source at Nintendo Japan.
Sometimes employees leave, Nintendo hires different marketing agencies or third party developers have no Nintendo projects going on so they have to pool resources together with others to connect the dots.
A rumor like Metroid Prime Remake could be something like this:
1. Next Level re-hires someone who worked on Metroid Prime: Federation Force before, their linked-in says: 'Working with the team on a follow up to one of my favorite GameCube games', his prior gig was ID Software or whatever
2. Someone doing QA says he's testing a classic Nintendo FPS game that's decades old at this point
3. Marketing agency X claims they're doing an online ad campaign for a highly rated space-themed title this holiday
4. The voice actress of Samus is working with Nintendo on a new project
5. METROID PRIME is listed for December in the database of a company that prints licensed T-shirts
All these things "could" point to Metroid Prime. But it turns out they are 1 = Mario Strikers: Battle League DLC, 2 = GoldenEye, 3 = Mario + Rabbids, 4 = Voice work for Xenoblade 3, 5 = they simply forgot to add the 4 and put in a placeholder date voor December