Microscopic wormholes are theorized to exist, but if they do, they likely formed during the earliest days of the universe during rapid inflation.Black holes can be microscopic according to current theory, so I'm not seeing a problem. Not every black hole they encounter needs to be a galaxy gobbler.
Later matter distribution and pressure of the universe doesn’t allow it anymore.
There is a small black hole 1,500 light years from us. It is 3.8 times the mass of Sol and, according to Einsteinian equations, it would be 15 to 24 miles across.
NASA - NASA Scientists Identify Smallest Known Black Hole
Black hole is closest to Earth, among the smallest ever discovered (osu.edu)
How small can a black hole be? | symmetry magazine
And this is why. Stellar black holes need a minimum mass to form when a star dies. We can guesstimate the lower limit but don’t know exactly.
The actual body of the black hole is a few km wide on that lower end.
That jives with the episode I think.
But the problem is the accreditation disc, if it has one.
Not sure how wide that would be, cause it‘s still the mass of a star that it orbits around.
So either it must be very far out and not be very dense to give such a spectacular light show, or it‘s very close but extremely fast orbiting, like relativisticly fast. The hard radiation from that would cause a major problem for the Enterprise and her little stunt.
Imagine the mass of what is effectively a small star (the brown dwarf) being accelerated to near light speed and bombarding the ship.