> Or I could ignore what are obviously semantics following a clear retroactive change in US policy that affects criminal law.
You can ignore whatever you want, but it doesn't change what the words in the Constitution meant in the context they were written, and have been consistently been interpreted by the courts to mean. Arguing that something is unconstitutional just because you've invented an entirely ahistorical set of definitions of the words used is, well, not all that unusual, but still not particularly interesting.
> It doesn't have to be as complex as you make it out to be.
Its not at all complex: A government action is an ex post facto law if it criminalizes, or increases the criminal sanction for, an event that occurred before the act occurred. Even by your own characterization, the only thing this might criminalize is a refusal to pay the newly-calculated tax after the policy. So, no ex post facto law.
You can argue that it is undesirable for other reasons, but you can rest on the Constitutional prohibition of ex post facto laws to do that when its not an ex post facto law.
> The NYT even called it a retroactive policy change!
Its obviously a retroactive policy change, in that it applies to IRS assessment of taxes for prior tax years.
No act that was committed in those tax years becomes criminal because of the policy change, so its not an action which retroactively criminalizes an act or increases the criminal penalty for an act.
"Retroactive policy change" and "ex post facto law" aren't the same thing.