It's way more likely that such players have simply been playing for much longer than you. It doesn't take much to grind out your dailies every three days, and unless you're exceptionally good at arena they will be your main income source. That's just how HS is structured, except for a small percentage of arena nuts someone who plays for hours on end won't get far ahead of a player putting in a handful of hours each week. The majority of players doesn't invest much time into watching streams, studying guides or frequenting threads like this one, they're content with having fun with their own wonky or cheap cookie-cutter decks and get new cards.It does. The guy also had a different card back than I've seen. But he wasn't super good or decisive. Maybe he just spent a bundle of money.
So don't assume they spent loads of money just because they have some legendaries, it usually isn't the case. Often like you mentioned their card back at least gives you an estimate of how long they've been playing.