Well, but it's not about whether someone can or cannot do something. Since you seem experienced with these devices I'm just asking if there's any technical reason why this might be a bad idea other than the fact that it just doesn't seem like a very professional thing to do. Like for example even though I'm far from an expert I know Raspberry Pis would be awful for any commercial application because they are notorious for killing their SD cards rendering the device useless.
People do whatever they want. It doesn't have to make sense.
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.
The Teensy wasn’t engineered, tested, rated, or certified for any sort of continuous duty, let alone within a pressurized O2-enriched environment (assuming it was inside the vessel), especially not within deep sea Helium-enriched environments (that have been shown to break things like MEMS devices), and present unnecessary risks for an entirely inefficient choice (see: comment above, Teensy’s are ~$30-40/ea where small PCBs populated with the same circuit features can be had for under $20).
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).