1) They don't have an elegant way to account for the combination of inflation and returns, in my opinion, that makes it difficult to say too much about this.
2) Are you already at your portfolio and actively withdrawing? Specifically, their calculator is set up around age, not stash size, so if not that might be messing with things. So, if you expect to have $1,000,000 in 10 years and withdraw 4% of that, they are accounting for the fact that we might see a market drop in 9 years but you retire anyway. That's different than working an extra however long to get back to 1 million then retiring.
3) They use monte carlo simulations. Some people have icky feelings about this. Not "30% success rate at 4% withrdawal" icky, but there can be meaningful statistical discussions about the validity of this approach and why it might be a little pessimistic. If you aren't already deep in understanding this mathematically or particularly interested, I'd ignore this bullet point because it doesn't matter really.
