Yes, it is the approval rate that matters, not the disapproval rate. The approval people are the ones who voted for him and which the republicans need to keep happy. If grump loses them, then those republicans will feel free to turn against him.
That said, this is only the 3rd time the approval has dropped below 40%, its never stayed below 40% for more than a day. But the last two times it dropped below 40% donald grump came out with a big noisy distraction. This time he hasn't been able to pull that off. We'll see in a few hours if the sub-40 level persists for a second day - if it does, that will be a first. Still just a babystep though.
ETA: Today's poll result: 39% approval rating. Two consecutive days below 40% Hope the FBI testimony today sinks it even further.