It never hurts to have "dumb" trading partners, but many trades are win-win. Teams may be loaded at some positions and weak at others, or may be all pitching or all offense. If a league has five or six unmanaged teams, those teams can essentially freeze the players a rebuilding team needs. In the two leagues I'm involved in that will be contracted, for example, I counted fifteen players that are genuine stars and over forty solid players that will be available in the contraction draft.
In several cases, the quality players are duplicates at their position for the unmanaged teams. Were that team managed, an equitable trade would be possible that could help both teams. As long as they're unmanaged, neither the unmanaged team nor other rebuilding teams in the league are able to get more competitive through trade.
That's the logic of the comment--unless I'm missing something here.
Dave N