The US were not thrilled about it when it was being constructed, obviously, but this was normal tensions towards Russia, prescient in the end but here we are.
A month or so later, Russia launched the 2022 offensive against Ukraine, and there was no longer any question of NS2 entering service because it was clear to all that the preconditions for Germany's rescission of approval for the pipeline had been satisfied. With that context, Biden's answer is best understood as him being quite confident in the quality of US intelligence that Russia was planning an imminent invasion of Ukraine that Europe was assessing as faulty. So while Europe was interested in the question of "what if Russia doesn't invade Ukraine?" Biden's answer was (in not so many words) "I'm not contemplating that scenario."