A person builds out their circuit using hardware they can solder/wire-up by hand on a workbench, maybe even with relatively-giant solderless breadboards, to prove the concept and the general design.
And a dev board can be great for spinning a few prototypes. It's quick to get started (code can begin being tested on-chip after just plugging in a USB cable), and to try different things and to make (and correct!) mistakes. (Blow up a Teensy? No worries; just grab another from the drawer, try not to make that same mistake again, and keep moving -- no esoteric soldering required)
But when the design is finished-enough and it becomes time to spin up custom-built PCBs for a final product that will be sold, a separate dev board like a Teensy tends to lose much of its initial charm.
Instead, it's more-typical just put the microcontroller IC plus whatever supporting hardware is necessary for the overall device's actual functions on the main board. Don't need USB, or an Ethernet PHY, an LED, a button, or a separate voltage regulator? Want more or less flash? When including the MCU on a board of one's own design instead of a kitchen-sink dev board, one is empowered to use exactly the parts that are required.
This can save a substantial amount of space and greatly improve the flexibility of the layout, while also improving mechanical and electrical robustness by having fewer connections between the MCU and the world around it. Plus, fewer parts tend to be less costly than more parts are.
(But again, it's not always done this way. This camera from the submarine is an example of one instance where the whole dev board was put inside of a finished product. Sometimes that's a good idea, and sometimes it isn't. I'm not attempting to suggest that it was or was not a good move in this instance.)
And I don't know when that camera was manufactured or designed.
But these days, it's possible to get even hobbyist-quantities of custom PCBs delivered with difficult-to-solder ICs installed from sources like JLCPCB.
(Depending on the features and functions wanted, it doesn't take a whole lot of extra parts to get an MCU to do its thing: There's not a ton of parts on a Teensy to begin with.)
In this context, all people are free to do whatever they want. It is beyond me to suggest that any person cannot do a thing.
I’m probably not as qualified as the person you replied to, but that’s my intuition as someone with a passing familiarity with electronics engineering (I have an associates degree in EE).
Perhaps disturbingly: I even know of one bit of critical public safety communications infrastructure that is is expensive, low production volume, and has a Raspberry Pi 3 embedded inside. I won't name names because that's getting a little too close to home for my liking, but I was quite surprised to find this inside of a very nice waterproof box with chonky, expensive, olive-colored milspec connectors to connect it up to the outside world.
Which, well: Yeah. There's a ton of good reasons not to do that. But building a whole Linux system on a custom board using individual parts is hard, so it can make sense to buy someone else's work instead.
Except... that's what the CM3 is designed to provide, including on-board eMMC instead of an SD card. I'd not have been surprised at all if there was a CM3 in there, but there is instead an entire Pi 3.
But MCUs, like on the Teensy, aren't like that. They aren't hard to integrate on a custom board like the Broadcom SoC on a Pi 3 or CM3 is.
The primary purpose of an MCU is not to be stuffed onto a dev board like a Teensy, but instead to be stuffed onto the board inside of a microwave oven or an air fryer or a fancy remote control and be easy to interface with other things and to program.
It really doesn't take much to get them going: Some require external ROM or flash, but a lot of them have internal flash memory and only need power and programming pins wired up to let them run code and do whatever IO is needed within a system.
This camera already had at least one very custom board inside. It could have integrated the MCU, as well, instead of the kitchen-sink Teensy.
Doing so is not just style points; it's quite often easier, cheaper, and more flexible.
This allows a person to use all of the IO pins on the MCU to do stuff with, instead of just the functions that the designer of a dev kit decided to build out through whatever interfaces they decided to include.