Credibility is a spectrum. If you're a highly specialized rider, who only really excels at GC's in mountainous and tough stage races, it's a much more credible situation than a rider who excels at genuinely everything, from riding Grand Tour GC's to beating heavy watt machines in spring classics. It's not a complex or difficult logic. It's about what is feasible and credible. Being a highly specialized rider? That seems both feasible and credible, because you're focusing all your efforts on a very specific kind of training and racing. Being the best at everything? That's neither feasible nor credible.
Look at Wout. He's such a ridiculously talented rider that 40 years ago, he would have been a Bernard Hinault, a rider able to win both Roubaix and the Tour. Today? It's simply not possible for a rider like him, because the sport has become so much more professionalized and scientific in it's approach, so whenever there's a reasonably good climber it's not difficult for his team to develop that specific skill set through a minute, highly detailed, scientifically backed training plan. That wasn't reality four decades ago, where the approach mostly was to train as hard and for as long as humanly possible.
Logically, if you want to compete in races like De Ronde and Roubaix, in order to reach those peak power wattages, you're going to have to sacrifice some of your climbing ability, in simple terms because you need to be a bit heavier. That's not the case for Tadej, though. He still easily outclimbs the most specialized of climbers, truly lightweight mountain specialists like Mas, Carapaz, Landa, Gall, and, of course, on numerous occasions, also Vingegaard.