How could scribes and Pharisees "shut the kingdom of heaven"? (Matthew 23:13)

I asked this question on the hermeneutics SE but I was told it would be better to ask here. What I can say is I'm not looking for an answer explaining that salvation is through Christ only, because it is obvious and it was even my assumption for this question. What I'm wondering is more about what Jesus actually meant if we know He is the only way of salvation. In Matthew 23:13, we can read: >But woe to you, scribes and Pharisees, hypocrites! because **you shut the kingdom of heaven against men**; for you neither enter yourselves, nor allow those who would enter to go in - **Matthew 23:13 (RSV-CE)** In Catholic, Polish Bible "Biblia Tysiąclecia", there's a commentary to this verse (translated to English): >By imposing excessive requirements around the Law, **they made it impossible for people to observe it, thereby closing the way to salvation**. They also bear the greatest blame for the people's unbelief in Jesus the Messiah. Is this commentary accurate? I'm asking because in my opinion, someone could conclude from this verse that the Law could've been observed in a feasible way that leads to salvation which we know is actually impossible because humans are not able to observe the Law entirely and perfectly (that's why Jesus, who can do that, had to redeem us on the cross). And also, would observing the Law in a hard way be considered a sin if it "shuts the kingdom of heaven" or not so much sin as it leads to commiting one? If it is, who is actually responsible for that sin? Were people aware of it? If not, why would God close the heaven for such people if they did it unintentionally? Or were they kind of deceived, so both deceived and deceiver commited sin? The only interpretation that comes to my mind is it refers to observing the Law before Christ's death, but still those people couldn't observe the Law perfectly and needed redemption on the cross. Maybe it is just about observing in the right way as much as possible, not observing perfectly and entirely?