Think of cell towers or wind power turbines: they both are primary hacking targets in today's world, and they are placed in the wild, in uncontrolled and unprotected locations. This means more or less anybody can just walk by, temporarily cut the power source, take the harddisk out, plug it into their hacking laptop, install an OS trojan on it, place it back into the original device and restore the power. From the PoV of the cell company or the power company this was just a short power cut, and nothing changed. I reality the system was just hacked. And in order to protect yourself against that OSTree can't help you, because disk accesses aren't validated. The only validation takes place during downloading. dm-verity OTOH will protect every single access, and if deployed properly then such "offline" modifications to the OS will result in the device not booting anymore, which is much preferable over accepting that the device was hacked with no scheme to detect it.

And it's not just cell towers or wind power turbines: pretty much any device which is around people not unconditionally trusted needs to be protected against such offline modifications. In fact, if people today build cars, TVs, surveillance cameras or anything else like that and do not deploy dm-verity in some form to make sure the devices cannot be modified offline without noticing are just participating in turning IoT into Internet of Shit.

He probably means modification by those who have physical access, which means often the users, but sometimes they are not the owners.

If you have devices like cable box or water meter, the real owners do not want you to modify the device. That's where mechanisms like dm-verity step in.