Sealed back speakers are piezo type, or, there is a port. I'm a retired high-end sound and lighting guy
For the cone to move without resistance there needs to be air flow.
It is true that most sealed-back speakers today are piezo, but I have both tweeters and mid-range cone speakers with sealed backs from the mid-70s. They were used to prevent interaction with large, long-throw woofers in base reflex designs.
That aside, stuffing a small speaker enclosure is really no different than putting batting inside large enclosures; it alters the frequency response.
Personally, it's not worth the effort for me as I use headphones unless there are visitors in the shack.