I am sure that many IEMs don’t have crossovers, at least crossovers in circuit form, which is certainly the case with many Chinese brands.
However, you can use an enclosure to tune a driver (which I am sure is what Campfire are doing with their T.E.A.C). By using different enclosures (or chambers in this case) you can help a driver to roll off at a specific frequency, basically giving you an acoustic frequency filter (high or low).
In the case of speakers, it is much easier to use filters (high pass, low pass, band pass) as there is plenty of space. In an IEM it is pretty incredible that such small spaces can be used for tuning.
Also, when I said “any multi driver set up” in my previous post, that is not completely true. I should have said that any multi driver set up that is not using the exact same drivers.
For example, if you have a 3 way speaker (let’s say a 10" a 5" and a 1" driver) and each driver is reproducing different frequencies, then they should be divided in some way. If, for example, you have another 3 way speaker, but in this case you are using a 10" 2x4" and a 1", the two four inch drivers are reproducing the same frequencies and do not need to be divided amongst themselves. They should, however, be identical drivers and should still be divided from the 10" and the 1".
If the drivers are not identical and are still receiving the exact same frequencies, they may turn out to sound good together but it is luck of the draw (or a lot of trial and error)