Well, one could see mountains in the Mirrormere too, which is what sparked the question in the original post of course (though I note the 'wink' in any case).
Now I don't agree it has anything to do with Gondolin (though it's interesting to note that Tumladen was once a great lake) but had this been an Elvish site, and a connection to ancient Gondolin had been made in an even tenuous way, the fact that Beleriand was drowned in the past would not necessarily mean a vision of the mountains about Gondolin was impossible.
The Mirrormere cannot be explained by science as far as I know (a very good thing in my opinion), but as Virumor
already posted, 'encircling' mountains is arguably just Tolkien's choice of wording here. With respect to the passage Grondmaster
brought up, Hammond And Scull note in their recent Reader's Companion:
'David Cofield comments in 'Harbringer of Fate: The Eclipse of 3019' in Beyond Bree, October 1993, that it was 'an ancient tradition that observers at the bottom of a deep gorge, cavern, well, or chimney are able to see stars in the daylight sky', but recent research has shown 'that the glare of the sun and the atmospheric scattering of its light make it impossible to see any star (except the sun itself) from the earth's surface [in daylight] with the naked eye' (p. 3).
In reply, Carl Hostetter comments in a letter to Beyond Bree for November 1993 that even if the 'chimney effect' is disproved, 'it is clear from Tolkien's own explanation that he believed [it] to be an authentic phenomenon, which is really all that matters' (p. 8).'
Anyway, if the reflection of stars in the Mirrormere is that of Durin's time in a distant past, so too could some 'mystic' body of water reflect other things... even things already under the Sea with respect to time. But again, I agree we happen to have the word 'encircling' here rather than some cogent connection to Gondolin.