Haven't listened to it yet but Mike is incorrect, some IEMs use dynamics as well.  It's okay though, Mike is a really cool guy and more about the music than tech.  I'll give it a listen later and throw in my 2 cents.

Never seen an actual universal IEM have a used driver from a hearing aid. Hearing aid drivers, even ITE ones, tend to have a narrow bandwidth, since wide bandwidth would only increase noise ratio and old people do not hear high frequencies well anyway.

The part about all tiny details mattering is true to an extent - but not as much as they make it. The driver still matters the most, then the dampers or filters, venting scheme if any, crossover or impedance and then the shell (unless it's vented, then it matters more). In CIEMs, tubing length tends to matter in matching the drivers, material usually much less, but still.

Few drivers are built from the ground up just for music. Some of them have been optimized for music, but really hearing-aid drivers can be used for the same purposes.

A lot of these companies have very strong backgrounds in hearing aids, and as my friend (who has way more experience with this stuff than I do) was telling me the other day, Sonion has typically pushed for the pro in-ear market to differentiate itself more. In other words, the designed for music thing is honestly just marketing for the most part.

It's usually not old used drivers though, just new ones. Hearing aid companies just have an open deal line with some BA manufacturer, like Sonion or Knowles, and have experience working with ear moulds and simpler driver setups, so offering a CIEM is a logical step to expand the market. They should also know how to tune the drivers - much like hearing aids are tuned, except those are usually personalized.

Remember that companies such as Audeo (recently out of IEM business) or Westone are also hearing aid manufacturers.