And for all we know, the Romulan cloak in TOS and TNG is based on the very same principle, and none other exists in the Trek universe - save for the phase cloak, which is the one and only innovation in the field, and justly revered for this.
This doesn't explain why Spock would remain ignorant of Xyrillian and Romulan invisibility as late as the 2260s - never mind Suliban invisibility, a technology Starfleet itself exploited to notable tactical gain. Sure, invisibility might be a major military secret, and its use generally so limited that civilians need not learn of it. Starfleet thus might practice self-censorship and only inform certain key officers of this technology and its past uses. But nothing in TOS or other Trek makes this particularly likely; invisibility is quite commonplace there, and likely to be encountered by all sorts of spacefarers.