In fact, the Luton team is relatively weak. The team was in the relegation zone. And won only one match out of 11 matches. It was unthinkable that Luton would pick up a point against Liverpool. But we have to admit that Luton's players performed very well in this match. It is true that they played defensively, but they managed to make some excellent attacks. Liverpool played completely attacking strategy. Luton's players seized the opportunity. Luton's players made some great counter-attacks and managed to score a goal in the 80th minute. But they were not so lucky. After 90 minutes, Diaz's goal brought the match back to parity. Despite not getting 3 points from this match, Luton's performance was very good. If they can perform like this, they can stay away from the relegation zone.
You are right that Luton are frankly a weak team and it is likely that Klopp expected an easy win in this match, but in the Premier League you cannot underestimate any team, and Liverpool made this mistake. It's bad because Liverpool miss the chance to get within a point of City. Aston Villa also unexpectedly lose and lose the opportunity to take advantage of the fact that Arsenal lost points, they lose the chance to climb into the top 4.