True, but that doesn't somehow make running fewer trains more expensive than running daily service. The only way Amtrak has to control costs on an ongoing basis is by trimming operational expenses.
While that's the only (or at least primary) way they can control costs, they can also work on the other side of the equation (increasing revenue, or at least slowing decreases in revenue.) While economics during a pandemic are a huge unknown, in non-pandemic times a cut to 3-days a week (as done in the 1990s, I believe) wound up losing more money than was saved with less costs, and ridership dropped quite a bit as well. While someone who absolutely wants to take a train (railfan, once-in-a-lifetime trip, etc.) or absolutely needs to take a train (only intercity transport option, medical issues with flying and can't drive, etc.) will wind up making it work, a lot of ridership (at least non-pandemic) is semi-choice travel. They might prefer the train, but if the schedule doesn't work they'll take the bus, drive, or fly, or maybe they'll only take the train one way instead of both. They might also just not consider it if they can't keep track of what days the train runs easily.
Expenses also don't drop evenly, even for the cost to run the train. Sure, you don't have as many conductors or engineers on the trains, but they'll sometimes have longer layovers that they need additional compensation for (for example, they'll have a day or two on their layover in Minot, instead of running the train back later that evening.) I also don't know how the contracts work with the railroads, but it's possible that it's a simple "pay $X for daily service, but if you decide to run less trains than that you still need to pay $X" or the drop in cost isn't directly proportional to the trains that are ran.
Simply stated, while it costs more to run trains 7 days a week, you generally get enough revenue from the additional sales (since not everyone will move to a different day) to make up that difference. Now, during a pandemic, who knows, but I think it's worth having Congress fund Amtrak enough to at least keep service at a 1x/day level throughout the pandemic and the recovery from it, simply to make sure that everything's still there when we're on the other side of the pandemic.