Giving some charity to the claim, I'm assuming it meant on devices you plan to dispose of.

I don't see how throwing it away improves storage capacity in a way that would invalidate GP's comment

The assumption isn't infinite capacity. It is recovery of the last data.

Is that fundamentally different from the question of whether overwriting data can make it unreadable? If yes then you can eventually make the "last data" unrecoverable by overwriting it. And if no then presumably you can recover anything previously written to the disk, ergo infinite capacity.

Presumably the original comment meant that you can't guarantee the original data is unreadable with any finite number of overwrites, not that you can always read the original data after overwriting. I'm not saying that's true, but it's a logical interpretation that doesn't imply infinite storage.