So: “Biden paid more and we don’t have enough info to say why, let alone whether this is somehow unfair.” Is that it?

It's basically this. Biden paid more in the past, but at some point in September things crossed and now Trump is paying more.

Audience targeting also seems to have a big impact on ad cost for FB, so the only real conclusion to be drawn is that the ad rates changed, possibly because of different targeting parameters. That feels very similar to comparing costs of televised ads while not knowing what time slots were involved.

That said, one ad marketplace is far more opaque, so it's hard for an outside observer to accurately gauge fairness.