As many have said, not all altcoins become more expensive when bitcoin becomes more expensive. This is logical because everyone is trying to have time in the new run of bulls.
Because most altcoins are crap in the first place. How can a particular altcoin without no reason to exist will rise its value? Even if bitcoin is in the upward trend, most altcoins will not follow because most of them were created for dev's profits. So if people will see the emptiness of that platform, they will not be interested in trading them. So going back, I don't think the price movement of btc is fake bull run. The bitcoin's time is already ripe for this. And not driven by the hype but showing the real value in the market.