Is this a mistake?

In this game, I got flagged when my opponent had insufficient material - is this a mistake, or am I missing something?

But it is impossible to get to that position - for example, what was white's last move in that scenario

White Kb1
Black Kb3 ,Nb5 ,Nd4

With black to play it plays Na3 check
After that white should not go to a1 but if does Nc2 is mate

It's impossible to FORCE mate in this position. Assume it's white's turn, with a black knights on c3 and d4 and his king on c2. White's king is on a2.
If he plays Ka1??, black plays Nb3#

You seem to be missing the point, OP. It does not have to be forced--merely possible. ;)

@rubixcube9
Read: lichess.org/faq#timeout
On that basis, 2 knight checkmate is possible. So, the lone king can at most draw when other side times out or by move rule.

#1
FIDE rules never talk about such a concept as insufficient material for mate. The closest concept in FIDE is a dead position.

"5.2.2 The game is drawn when a position has arisen in which neither player can checkmate the opponent's king with any series of legal moves. The game is said to end in a 'dead position'. This immediately ends the game, provided that the move producing the position was in accordance with Article 3 and Articles 4.2 - 4.7."
- handbook.fide.com/chapter/E012018

As people have pointed out, the position in your game is not a dead position.

There is also nothing in FIDE that corresponds to the USCF rules on "insufficient losing chances".

Curiously, before black took the pawn, it _was_ possible to force the mate.

You should watch all the possible movements, and if the moovements were obvious you shoul do an strategy for do just forced plays