Indeed, this clock, which uses iodine absorption cells to provide the reference frequency, is one of the kinds of already existing portable optical atomic clocks to which I was referring.

The best laboratory optical clocks, which use ion traps or optical lattices with neutral atoms, have a higher accuracy by up to 6 orders of magnitude, which makes much harder for the system that stabilizes the length of the laser cavity to keep up with it.

Minute length changes that would not matter for a less accurate iodine clock would cause unacceptable frequency shifts in a SOTA optical clock. Therefore such optical clocks are much more sensitive to their environment.