He would insist that crowding out never happens. Government spending does not reduce private sector such, it is not true that if government tries too much then we lose that private output or effort.
In that case MMT suggests that increased tax charges might be required for two potential reasons. One is simply to control inflation. The other is that if a government still wishes to pursue what it thinks socially desirable activity in that situation then tax can be used to reduce private sector demand in the economy so that economic capacity is made available to ensure that delivery of the government’s own objectives is possible without over-inflation of the economy resulting.
Which is, government might decide to tax more in order to prevent that crowding out that never does happen.