Yeah, Anthony Grimani knows his stuff.

The whole idea of the aimable tweeter came up because there was no hard number on the dispersion of the in-ceiling M3, and elsewhere it mentioned that if you don't have a wide dispersion overhead speaker, that you need to angle the speaker towards the listening area. Well, since that would mean a much more complex M3 housing design to be able to angle the entire speaker (woofer and tweeter) it was mentioned that being able to aim the tweeter (where most of the frequencies that are easily "locatable" to human ears comes from) would be a positive step. Also, if you have lower ceilings, it was determined by others that had done testing on it that if you had a lower that recommended ceiling, you would put the speakers further away (further in front, or behind, and further out to the sides if possible) to allow the speaker to disperse the sound more, but then you would have to be able to point the speaker towards the whole listening area. So you are using speaker aiming-ability to compensate for the lower ceiling by pushing the speakers further away (like if you had a taller ceiling).

So in an ideal situation (correct ceiling height and correct speaker dispersion), you would want the speakers pointing straight down as spec'd by Dolby. It is when you are trying to overcome some limitation that things get a bit "wonky."

On a particular DIY speaker site, a very popular ceiling speaker option for Atmos is an on-ceiling (basically an on-wall speaker) that it's cabinet comes with an angled option (as well as a regular box that they call their "Atmos" box). I don't know what the dispersion is for the coaxial speakers that they use, but they offer both options specifically for Atmos due to demand (their words, not mine).

Farewell - June 4, 2020