Msgr. Lefebvre goes from affirming that the Pope can be heretical as "a private doctor or theologian", to treating of "to what extent Pope Paul VI willed to engage his infallibility....". But this is a jump from one subject to another;
It's setting up a false dichitomy, without a middle ground. If the Pope is not engaging infallibility, then he's teaching as a private doctor. With all due respect to Archbishop Lefebvre, that's utterly absurd. When a Pope issues an encyclical to the Universal Church, he is not acting / teaching as a private doctor, but as the teacher of all the faithful, even if he's not engaging infallibility.