I think the key here is qualification - Robert Morris avoided being wrong by not stating things unqualified unless he was sure of them, whereas the failure mode for rationalists is not expressing an idea at all unless fairly sure of it.

We want ideas to be shared before they're well-supported, because discussion is generally the best way for them to find support (or disproof) - we just need to signal the uncertainty when we introduce an idea.

It's much like what I've been taught in analytical chemistry - every number has a stated uncertainty associated with it.

It can easily be both; and, while not speaking unless very confident may not be terribly harmful by itself, the perfectionism that it's an example of is a very characteristic failure mode of rationalists.