There is an aspect of seeming illogic I want to call out here. Maybe it's just my lack of understanding.
Assuming the IRS is underfunded, saying they focus on low income people as a consequence, when it would be more lucrative to target high earners, makes no sense. The claims could all be true, but they don't appear to be connected.
Why would the total amount of funding force the IRS to spend less on individual cases? An alternative hypothesis might be that it actually is more cost effective to target people with less money, for instance, people paid mainly in cash tips, but I don't have any idea if that is plausible, I'm just saying the narrative doesn't make sense to me.
In general, targeting rich people seems obvious because rich people "have the money", but mathematically, it depends on how many there are when you multiply it out.
Anyway, I am very open to the idea that the IRS is being stupid and counterproductive, or politically avoiding prosecuting the rich, but I think it raises the question of "why" that is not answered by saying "underfunding". They still have funding to go after one or more high earners, right, so the total amount is irrelevant if it's taken as a given.