I can definitely foresee a future, based on governmental record in publicly-facing IT systems, where every single word of that statement is technically 100% true.
First part is all but tautological - Of course it won't s not free to build or operate; and thus it wouldn't be "free for taxpayers" in the most technical sense (this doesn't mean it wouldn't be excellent value or save money overall or even cost more money but save it where it counts).
It's the last sentence (unnecessarily cost taxpayers billions) that could go either way. This could in principle end up an efficient and effective, easy to use and helpful well performing system that's a brilliant investment of public money.
I hate to say it though, but it's also extremely possible it would cost a lot of money with nobody using it because it sucks.
Besides, it feels like in the USA, IT systems are only a part of issue with tax system. Actual tax code and submission options are the bigger part.