And there is precedent in TOS for Federation enemies to think that, since the Romulan Commander in Enterprise Incident believes it too.
One might say that many species are in awe of the Vulcan alienness - their frankly quite exceptional self-restraint, their scary mindmelding skills, their reputation as fierce fighters in the past, their no doubt fascinating sexual perversions. People who would consider a Bolian or an Andorian just a fellow member of their own species with a particularly ugly face, and not trust him beyond how much they'd trust their weird uncle or lazy son-in-law, would believe anything
And it is logical to think that Romulans would have a particularly severe blind spot in this respect, as their propaganda machine would have been feeding them "Vulcan otherness" for at least two millennia already. They might look down on enemies in general, but after such overexposure, the intense "Vulcans are different" propaganda would backfire and make them look up