It's not invulnerable at all. If anything the concentration of satellites at one altitude makes it more vulnerable because debris would spread out (think of "spread out" in a "shotgun" way) and threaten the other satellites of the constellation. These satellites move at speeds of kilometers per second, all it takes is putting something in their path to obliterate them. That could be a missile with or without ("hit-to-kill") a warhead or the debris of another satellite.

(Edit: keep in mind that Starlink operates on dozens of different orbital planes all at the same inclination and (approx.) same altitude that all intersect twice over the course of an orbit (which takes approximately 90 minutes). Pieces from the debris cloud would be at risk of hitting another satellite at every intersection at a high relative speed. It's not a low risk of damage once but a low risk of causing damage again and again until the debris has decayed enough).

If "Russia" (Putin) would like to disable Starlink satellites, I would guess that he easily could. If he wants to destroy an American-owned constellation and most likely raise the conflict to a whole another level is a different question.