The fact that AAA corporate bonds pay more than government bonds is almost always (maybe even always) true. They are considered riskier than government bonds, therefore they pay more.
The key words there are “tax-free” which most likely means municipal and other government entities bonds. Of course they can pay more, they get the interest from citizens.
That is what makes them so bad for the public. Sure they will pay you more because it comes from a bottomless source of returns; the common citizenry.
It actually shows the gall of our politicians and government administrators on all levels. “Sure we can pay that sort of interest, who are they going to complain to?”