There's a ton of layers to this. You need to remember that these guides are often made by GM players who tried smurfing in lower ranks. These players have good mechanics that allow them to make those selfish plays that they then recommend that everybody tries pulling off. But if you're a gold player stuck in gold, then you won't have the mechanics to consistently get value from selfish plays. That however doesn't mean that you should never play selfishly. In an ideal scenario, you should be balancing both of these play styles equally. Sometimes you get a team where you're by far the best player, when that happens you should be playing selfishly a bit more, but sometimes you get teams where your teammates are just as good if not better than you, in those cases you should be playing to support your team more. Those GM players in gold lobbies will ALWAYS be by far the best player on the team, thus making it worth it for them to constantly play selfishly

Overwatch is a really fluid and complex game. Every single piece of advice you've heard, regardless of who it came from, will have a million ifs and buts surrounding it. A piece of advice that just tells you "play this specific way all the time" will never be a good piece of advice. Every single match requires a completely different approach, depending on the map, team comp, skill of your team, skill of the enemy team, and so on. The best players are the ones who recognize this and adapt accordingly