The premise is that the tax rate of millionaires is of interest as distinct from the tax rate of the typical person because millionaires are considered as having an above average amount of wealth, i.e. the topic at hand is how the wealthy are taxes relative to the less wealthy. If inflation moves typical amounts of wealth closer to millionaire levels, the analysis no longer provides the same level of insight into that topic.

Of course it does.

It’s comparing the effective tax rate of someone who made $1m 1945 dollars against someone who made $1m 1980 dollars, and implying that the same should apply to someone who makes $1m 2024 dollars.

$1m in 1945 had the same purchasing power as $4.6m in 1980 - or $17.4m in 2024.

Those are not at all the same socioeconomic classes.