SEO is practical work; there's no absolute "true" or "false" in it. If someone says that SEO is just about building backlinks, it might work for them, but it may not for you. For instance, if someone succeeds using backlinks for a tool-based website that doesn't require much content, it might not yield the same results for a content-heavy site.
If you try the same tactic and fail, you might think SEO is dead, but that's not necessarily true. The key is to analyze your competitors, understand their strategies, and figure out how to beat them to rank higher. Success in SEO often comes from learning what works for your specific situation.
In SEO, your competitors are your best teachers.