goes down with distance by an exponential factor
(1/x^3)

Umm... those two things are not equivalent. b^(-x) would be exponential, x^(-k) is inverse-power for whatever k

The headline's confusing. If a losing bet is backfiring, does that mean it's now a winning bet?

Well, that's something I hadn't seen before.

It's not the word "lighter" that's the issue, it's the word "less". If I say something weighs 80% less, ... you know how much that is. 100% less, it weighs even less -- nothing at all. 500% less (i.e. 5 times less), suddenly it weighs more?

Neither, though I do watch a bit of Matt Parker on youtube, so it's a decent guess.

What, too soon?

heard that one earlier today ...

global warming is definitely something it makes sense to worry about and which there's still some chance to mitigate the worst effects of.

The sun expanding - or even the much earlier effects before that happens, as the sun gets hotter - will happen on such long time scales that there simply won't be any humans at all; most species only last about a million years or so, vastly less time than we're talking about.

We might well make the planet nigh uninhabitable in considerably less than one-millionth of the sun-being-a-major-problem time. It's like worrying about the bridge maybe rusting dangerously a few decades from now, while not paying attention to the truck that has just veered onto your side of the road and will surely hit you in the next few seconds. You need to take evasive maneuvers, not worry about the bridge.

basically will just die a boring death after swallowing all the planets in the solar system

Not all the planets, no. Mercury and Venus, sure. The earth's orbit will move somewhat further out when the sun expands, and probably won't be swallowed but it will at least be well baked.

Even a little button wear would be enough to pick between those three, so in a lot of cases you'd just need to try one of them.