Here's a crazy stat. Based on the number of discovered shipwrecks, we can infer that European shipping was 98% lower in the 8th century than in the 1st century.
And as with the Renaissance, you have to specify "where" as well as "when". For example, Scott Alexander specifies "The period from about 500 to about 1000 in Christian Western Europe was marked by profound economic and intellectual decline and stagnation relative to the periods that came before and after it." And commenters pushed back on that ranges for specific times and places within that window. The past is complicated, and Western Europe is a big place. That DOESN'T disprove the decline, just means you needs more nuance to talk about it.
@Quizmaster -- can you link the source on the shipping claim? I believe you, just curious what area they were looking at.
It's like crime statistics - often when levels of crime supposedly increase, it is actually that victims are more likely to report crimes / definitions of what constitute a crime change / police forces have become more efficient / etc
I've modified the link above, also pasted here for convenience.