It's a bit ambiguous. Up until ENT, we never got a scene that was demonstrably "unreal": for TOS, TAS, TNG and DS9, we just saw starships doing flybys of features known to exist in the Trek universe, and for VOY, we got some things that seemed to defy common sense (a ringed planet just a couple of hundred kilometers wide judging by that silly reflection on the rings) but were nothing to write home about in the Trek context. ENT gave us a string of images of which some looked as if "intended to be unreal" (the space station being built might have been a CGI demo rather than a timelapse document in-universe, say), but none of it was "art for art's sake" (but instead pretty images of existing things, even if those only existed virtually in that universe).
But then we got DIS, where flybys of giant eyes are unlikely to be footage from Starfleet sensors. And next we'll get PRO, where the flybys and -throughs are decidedly symbolic rather than realistic. So even by strict Trek precedent, LDS need not be interpreted as footage.
That said, nothing wrong with Romulans fighting the Borg. Everybody supposedly does that sooner or later anyway. And the Cerritos being there is legit, too. But where it gets unreal is all this happening twice, once with the Pakleds, once without...
'Course, LDS already features the heroes toying with simulated missions a lot. Could be this is indeed footage - of a holoadventure run by Mariner, who got bored with bravely fleeing mere Borg and upped the ante.
Timo Saloniemi