That's a good point, and I'm sure that would certainly be a problem with PageRank and similar ranking algorithms, but I wouldn't be entirely surprised if Google and other SEs have intelligently crafted a pre-processor that translates links like "kbin.social/m/lemmyworld@lemmy.world/t/34817/Is-Lemmy-Indexable" to the Original-Instance-Link (OIL, lurking Google devs feel free to steal this acronym) "https://lemmy.world/post/189226" so that relevant algorithms properly reflect the 'true' ranking of the information itself rather than the particular instance's... instance of it.

OStatus and Pump.io have been around for a while so SEs may (should) have already identified this problem and addressed it unless they've decided it's not important, not in-line with how their rankings are intended to work, or simply not easily solvable in some cases like I previously assumed. As Bjarne Stroustrup would say, "If you think it's simple, then you have misunderstood the problem."