I really don't get how at this point one could say on the one hand that the Pope's officially promulgated doctrinal teachings are infallible, and on the other that the recent Exhortation is heretical.
Somebody please explain this to me.
I'm sure there must be an argument out there but I'm at a loss as to what it might be.
Because papal utterances aren't automatically infallible. The popes aren't empowered by God to make things up as they go along. The duty of the Pope is to transmit and to guard the Deposit of Faith entrusted to us by Our Lord Jesus Christ, who is Head of His Church.
Vincentian Canon:
The famous threefold test of Catholic orthodoxy expressed by St. Vincent of Lérins (400-50) in his two memoranda (Comonitoria): "Care must especially be had that that be held which was believed everywhere [ubique], always [semper], and by all [ab omnibus]." By this triple norm of diffusion, endurance, and universality, a Christian can distinguish religious truth from error.
https://www.catholicculture.org/culture/library/dictionary/index.cfm?id=37106