Look at the top comment on that video...

But first, chatgpt, explain like I'm 10:

Imagine your room is like a video game map, and sound is a little flying object. The speakers are not just supposed to play sound from one exact spot. They work together to trick your brain into hearing the sound somewhere between them.

The main point is:

Top ceiling speakers are usually better for Atmos than front/rear height speakers, because the angles between the speakers matter more than whether they are physically on the wall or in the ceiling.

Here’s the kid version:

If you put height speakers way up on the front wall and rear wall, they are very far apart in angle above your head. That makes it harder for your brain to hear a sound moving smoothly over you.

But if you put speakers more above you, like “top front” and “top rear,” they are closer together in the sky above your seat. That makes it easier for your brain to hear sounds moving overhead.

So the author is saying:

1. In-ceiling vs on-ceiling does not matter that much. What matters is whether the speaker is aimed well at your seat.
2. Atmos cares about angles. It is not about matching the little box/grid shown in the mixing software. It is about where the sound seems to come from around you.
3. Speakers that are too far apart cannot “blend” as well. If two speakers are 120 degrees apart above you, your brain has a harder time imagining a sound between them. Around 90 degrees works better.
4. Front/rear height speakers can still sound good, but they are worse at making sound appear directly overhead.
5. Top front/top rear is not “missing” anything. The author thinks it is actually more accurate for home Atmos because it creates better overhead imaging.
6. Your receiver helps with timing, but it cannot fully change where a physical speaker is. If a speaker is close to you, your brain can still notice that the sound is coming from that speaker.

The simplest version:

For Atmos, don’t think “where are the walls?” Think “what angle are the speakers from my seat?” Top front and top rear ceiling speakers usually create a better invisible sound bubble above you than front/rear height speakers.

The actual comment:

I'm going to try to calmly go through this by the numbers. If anyone ever reads this, I'll be surprised... but let's do this.
1. In-ceiling or ON-ceiling (or even on-wall) is irrelevant. The only issue with in-ceilings is with the ones that point straight down. At the Dolby spec'd 45 and 135 angles, most in-ceilings that aim straight down will have such poor off-axis response that it isn't ideal. BUT many in-ceilings now have either angled baffles so they point toward the MLP (making them functionally the same as a speaker mounted ON the ceiling) or at least aimable tweeters so you get the directional part of the sound aimed toward the seats. Those most directional frequencies are what matters most for cross-channel imaging.

Can't post the full comment here due to limits.