Looking at the nearest Acela trains on the schedule, it's saving about 20 minutes (it's timetabled at 2hr 37min, and the ones immediately before/after are 2hr 56min and 2hr 59min.) There's probably shorter ones out there, but it seems fairer to compare it to the nearest timed alternatives.
That said, does the extra 20 minutes shave off enough to be fully time-competitive against the air shuttles? Right now, with traffic, from the Capitol to DCA is 12 minutes, and to Union Station is 6 minutes. LGA to Wall Street is 46 minutes, whereas Penn Station to Wall Street is 27 minutes (although only 17 minutes by subway.) Padding a bit for traffic, that's 45 minutes extra for travel (round-trip.) Assuming an extra 45 minutes for security formalities, and a 1:15 timetabled trip, it's already 2 hr 45 minutes for the air shuttle plus added travel time to/from the airport.
Perhaps being able to be just under instead of just over that will win people over, but it has to be enough and at the right time to win those passengers over. There might be enough latent demand that the frequency would fill up regardless, but I don't think the extra 20 minutes would be enough of a consideration to fill up a train on its own (the variability in travel time to/from the final destination would eat that up, I'd think.) That said, maybe this was one of the only ways to add the frequency at that timeslot (especially southbound you'd be hitting rush hour at many of the intermediate stations) and emphasizing the "nonstop" trip is just a way to make the added frequency look a bit more attractive than just noting that there's another train.