You're totally right — good catch. I actually just tested it myself: the leaderboard is one shared list, so if someone's real score is in the top 10, it'd show up the same way whether you're in a private tab or not. The "(you)" tag just won't show since a private tab doesn't know who you are, but the list itself doesn't change.
Turns out there's a separate issue causing this — looks like the current top of the board has some scores that aren't reachable through normal play (way higher than the game's actual payout math allows), so it's likely gotten a few fake entries. I'm looking into tightening that up. Appreciate you both digging into this!