Xiaomi air purifiers do exactly that. Go by hours used.
At least in the older Version 3 that was still the case.
Apparently people still buy them, because they are cheap. It's the same with printers - the initial purchase price is low, then the consumables are expensive.
People really need to include the running costs into their calculations. The best deals aren't usually the cheapest machines.
We don't know it said that - it could have been saying 'arbitrary number of seconds of use has elapsed/. You could be right, but we don't know.