You're not wrong, but this isn't really a security matter, it's an "apparent uniqueness" matter. Their goal, I assume, is to satisfy critics enough that a given petition's participants are sufficiently unique while keeping the barrier to filling out the form as low as possible. So they end up in a situation where neither of perfect, but they're both "good enough" for what the business needs.
I dealt with this in the anti-cheat space: my goal was never to remove all cheating, because that's too expensive (insanely so). My goal was to make the public believe they weren't playing against cheaters too often. If the solution was forcing the cheaters to perform at a level that was just below the most skilled human players, that was actually a success, because if the players can't differentiate between cheaters and pro players, then they can't effectively determine how prevalent cheating actually is.
Part of me hated that we had to treat it that way, but another part of me understood that if I pushed too hard on "eliminating cheating" my department would become more costly than it was worth and they'd pivot away from gameplay that needed anti-cheat at all