The writer is putting together two somewhat different ideas at the same time:
accounts by ... Marco Polo were widely reported
accounts by ... Marco Polo were somewhat distrusted
When the sentence is written like this, you can see that distrusted is acting as a passive participle, hence the -ed at the end.
The first idea- that the accounts were widely reported- is easily verifiable by consulting documents from the time. The second idea- that the accounts were somewhat distrusted- is possibly less well documented, and may be the opinion of the writer.
These two ideas are combined using if. The intended meaning is similar to but, with the suggestion that the information presented may be unreliable.
accounts by ... Marco Polo were widely reported, if somewhat distrusted.
accounts by ... Marco Polo were widely reported but possibly somewhat distrusted.