Amusingly, Star Trek is almost as strict about the difference between "freighter" and "transport" as the real-world navies are.
I have to admit, Scotty must have had a longer civilian career based on the above in some capacity. There isn't any other reason to distinguish starships from freighters and cruisers unless this is the intention.
Hmm... I just brought this ship up in another thread, so it's top of mind, but wasn't the Starfleet-operated USS Huron
identified as a freighter? If so, maybe they weren't as strict about this in the TOS era, and so it wouldn't necessarily imply Scotty had a civilian career prior to his Starfleet one. (To be fair, it doesn't rule it out, either!)