What sense would that make? It only makes sense that it was from the left, or from a ‘swing’ vote on the court.
The Alito draft given to Politico has a nominal date of February 10, 2022. The accompanying Politico story also indicated that at the time of its preparation (and, apparently, still today) Alito's draft commanded the votes of four other Justices (i.e., Thomas, Gorsuch, Kavanaugh, and Barrett) but not, apparently, C.J. Roberts. This suggests -- and, for what it's worth, subsequent alleged leaks have indeed suggested -- that Roberts was inclined towards concurring in the judgment in Dobbs (which is to say, throw in with the five Justice majority in upholding the Mississippi law at issue), while declining to join the majority in overturning Roe and Casey).
If this is indeed the current state of affairs among the Justices, then the loss of one of the five Justices currently in the majority -- e.g., with either Gorsuch, Kavanaugh, or Barrett (although surely not Thomas) going squishy and throwing in with Roberts' more "moderate" position -- would result in Roe (and perhaps Casey) surviving after all.
Under this (admittedly speculative, but nevertheless reasonable) scenario, the leak of Alito's initial draft, which on its face is styled as "the opinion of the Court" -- which definitively establishes that, when drafted, it was understood to command at least four Justices in addition to Alito -- could be seen as a strategic ploy to "lock in" the aforementioned "squishy" Gorsuch/Kavanaugh/Barrett. Otherwise, when the decision in Dobbs was finally issued, and one of those three had defected, it would be obvious which Justice had folded in the face of Roberts' pressure.
So that's at least one plausible explanation for why it would make sense for the leak of the Alito draft to have come from someone aligned with those on the Court anxious to see Roe and Casey overturned. I have no idea whether any of this is actually true or not, but I don't see why that should be so hard to understand.
Furthermore, the informational leak to Jan Crawford of CBS News in July 2012, following the first Affordable Care Act case -- where Roberts rather obviously switched his position at the last minute -- is generally understood today to have come from (an obviously incensed) Justice Scalia himself. This little factoid of Court history in turn provides a certain precedent for this action. Moreover, this perceived earlier betrayal on the part of Roberts would serve to provide a motive for action on the part of someone who didn't want to see something along those same lines play out again in a case of this importance.
Again, I don't see why any of this is so hard to understand.