The problem is that the EPL is overrated. I don't see any EPL clubs showing results that would confirm that it is the number 1 league in European (and world) football. Sometimes one club rises to the top level and can even win the title, but the same happens with clubs from other countries. PSG is undoubtedly a top club, so underestimating it was a clear mistake on the part of both bookmakers and fans (and maybe on the part of Liverpool players, although they are professionals and perhaps they understood everything correctly).
Yes English Premier League is mostly thought to be best club football league but it might not be as great as people think. Even though some EPL teams are very good for a while they do not always do well enough to be considered best. Other top European teams like Paris Saint Germain have shown they are also very good and competitive. PSG has done very well and people who did not think they were good enough were wrong. This shows that maybe EPL is not as dominant as people think.