This is false. These devices are designed to be running off a battery. There are no performance drops or compromises when on battery power (when I got my M1 Max I tested this because I needed to see it for myself - the performance does not change at all between the different power modes). In fact, some models of at least Intel era MacBooks actually need the battery to maintain power delivery and can be flaky when run with no internal battery (I have run some older MacBook Pros with their batteries removed due to swelling as lightweight servers).
There's tons of evidence that refutes this guy's claims.