The reasoning Billuart provides is identical to the reasoning provided by Miaskiewicz for juridical supplication in common error, for which he says there are no theoretical limits.  The idea is that someone who purports to hold an office (when they actually don't, but most believe they do) would have jurisdiction supplied to validate certain acts for the "common good and safety" of the Church, just as Billuart argues.  Why else would Biullart make the distinction that this is a special dispensation from Christ directly?  Keep in mind Billuart is writing prior to the codification of the Pio-Benedictine law.  At the time he was writing, it was still a matter of legitimate controversy whether or not supplied jurisdiction "was a thing," how it worked, under what conditions it worked, and so on.

If it's an incorrect reading of Billuart then it's incorrect, but let's not pretend like it's obviously incorrect.

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I realize he's not calling it supplied jurisdiction, but that's simply due to the lack of extant development regarding supplied jurisdiction among theologians and lawyers of the time, as I attempted to indicate in my previous post.  Why would Billaurt refer to what he's describing as something special if it were simply a matter of course that heretics (or heretical popes) don't lose their office?  Why is Christ's special intervention required if his position is (simply) that no jurisdiction is lost is in the first place?

Supplied jurisdiction comes from the Church, and consequently had the pope lost his, there would be no special intervention of Christ to supply what the Church could have supplied without His intervention.

Welcome back, Sean.