You saw it right. On the goal scored by Nick Suzuki, the Canadiens were playing 5 against 4, even though the team was no longer on the power play.
See the explanations by Jonathan Bernier, from the Journal de Montréal.
"(On Suzuki's goal) the Blues were then one man short even though Samuel Blais's major penalty had expired five seconds ago.
No, it's not because someone was sleeping on the Blues' bench as initially thought. In fact, with his team ahead 2 to 0, Drew Bannister, the head coach of the Blues, chose not to send a player to serve the penalty of the Quebecer forward, expelled from the game for delivering an illegal check to Jordan Harris."
Strange decision.
This allowed the Canadiens to be 5 against 4 for five minutes and five seconds, and it could have lasted even longer if Suzuki hadn't outsmarted Binnington.
"[The head coach of the Blues] deprived himself of the right to send a fifth skater onto the ice at the end of the penalty. The rule states that in such a case, one must wait for a stoppage in play to be able to send a full lineup back. A stoppage that never came." - Jonathan Bernier
This explains that.
It's good to know for the future, although we're unlikely to see this again soon simply because it's completely illogical as a decision.
It didn't cost the Blues the victory, but it could have completely changed the game.
