We are not talking about the large trunking measured in inches, there has never been a problem with installing singles there, thats what it is for. What we are talking about the questionable practice of putting them in the YT2 and similar sizes of mini trunking much loved by housebashers and data installers...
521.10.1 Requires the lid to only be removable by a tool or deliberate action* if it is to contain unsheathed cores.
*I'd interpret delibrate action as needing to obtain 'something' to pry the lid off, a knife, fork, lolly stick, anything. Most mini trunk you can remove snap on bends with forefingers, and the lid is easy to peel off. Whereas a 4x4 trunk thats in the Mains room, Manator, you need your biggest terminal driver to release the turn buckles