I don't think Anderson wants to keep the "old-school" or status quo dining service with wait staff, an on-board chef, etc. To him, he sees a mandate from Congress that says that food service cannot lose money. He also probably saw the current dining service and saw it as extremely labor-intensive for what's generally considered an okay, but not outstanding, product. Given that, and given how expensive labor is for Amtrak, he probably saw the contemporary dining service as a way to make the dining service "profitable" again. Having an airline background, he saw that meal service can be just as good, if not better, than the standard Amtrak diner with off-board prepared meals that are either kept warm or reheated on board, and considered that to be the best path forward. By cutting out labor (so costs can be saved there) and by not having public prices for the meals (so the entire cost of the meal service can be billed to the sleeper account without customers seeing "sticker shock") he sees this as the easiest way to fulfill the Congressional mandate to have dining service profitable while still keeping a relatively decent menu.
As for why he doesn't want to advocate for Congress to eliminate that mandate? Well, he probably doesn't really care too much about the experience of the dining car (seeing it as little more than a different way of delivering a meal, neither better nor worse than having a boxed lunch that's picked up outside of what food is offered.) He also has limited political capital to expend for Amtrak, and I think he wants to save it for something else, whether it be new equipment, route expansion, PTC funding, or something else. To him, it's not worth expending political capital to save the dining cars that he probably doesn't see as particularly special.
