You seem to be assuming the Thasian ship was zipping around invisible until they reached the Enterprise. There’s no indication of that.
I need assume nothing. The Thasians appeared out of nowhere right next to the hero ship, exactly like the Romulans.
It does not matter
how they achieved this. The only thing that matters is that Spock is familiar with the phenomenon itself. He has no excuse to go babbling about theoretical bending of light when there's nothing theoretical whatsoever about the ability of an opponent to appear out of nowhere right next to the heroes.
Now, it is theoretically possible that the Romulans indeed achieved this by bending light (even though Spock has no basis for speculating that, he may have guessed right, and his putative later sensor readings off screen might have confirmed that, although nothing in later Trek directly backs up the claim), and that the Thasians did not (Spock did get some sensor readings after the fact there). It doesn't make "Balance of Terror" any more excusable, though.
However, even granting what you say, the Romulans weren't really more advanced a race than humans or Vulcans, neither of whom had achieved cloaking tech. So, yep, for Spock it’s new and unexpected to see someone of the same technological level being able to render a ship invisible. There’s no continuity error here.
Why would the Romulans be on the same technological level with the Vulcans? If Spock is to be believed, the two have not met for ages - or at least the Vulcans know nothing of the Romulans. Later Trek shows that ages here stands for at least two thousand years (while also putting the lie to that), meaning the Romulans could have come up with immortality, warp 928345.47 engines and sixteen differently colored ways to phase-cloak.
What the heroes actually
can say about the tech level of the Romulans is that they used tech a hundred years prior in a war. Were they holding something back at the time? Were they on the verge of developing new stuff? They
have come up with the plasma weapon, meaning Spock would be wrong to assume they only have things the Vulcans could have.
Basically, "Balance of Terror" is the only episode from an alternate universe here. In all the others, the TOS heroes are familiar and comfortable with invisibility, and so are their predecessors and successors. Furthermore, everybody is comfortable with the idea that invisibility comes in degrees, from cheap visual illusions to the magical ability to disappear from every sensing method ever devised; they allow themselves to be surprised by better invisibility every now and then, but they don't disbelieve in it.
And everybody is comfortable with the idea of tech exchange, whether by trade or espionage or outright theft and conquest.
Timo Saloniemi