That's not a requirement to get a generic approved.
As long as a molecule has been approved before (through a New Drug Application), all follow-on drugs get approved through an ANDA (Abbreviated New Drug Application).
The real reason why inhalers are so hard to make generics for is that it's not just a drug - it's a delivery system, which is way more technologically complex than just producing a bunch of a chemical and putting it in pills.
Generic companies aren't interested in the investment. Having a factory make a ton of drug is cheap for the most part. You can put it in pills can cost a few cents per pill.
Investing in a new inhaler device is a major investment. Kind of similar to how generic injectable drugs is not as attractive - the investment is much higher.