He means the quality of trials rims, they're only held together with pins so are likely to come apart with more revolutions, and are not often completely true out of the factory. Mavic rims cost a lot because you're paying for a higher quality rim that will last longer.
Trials rims live a short life due to being battered and ground constantly, so it's fine to have cheaper, shittier rims because they don't last more than a couple of years and don't do many revolutions.