The charms were bugged to not work with traps/pets at the start of the season, but they work now.
Sunder Charms have been fixed quiet some time ago to work with summon type units. What really holds trapsins back in terms of endgame pvm viability is that those units still dont benefit from enemy resistance modifiers on your gear. You could for example build a very strong fire-centric blade/trapsin hybrid if they fixed this issue. And not like its a huge effort, Hydra sorces suffered from the same issue and got it fixed already.
To be clear, if you read the guide, it summarizes the issue correctly. I play sorcs, so my error. I've updated that sentence in the news post to reflect that the issue with Traps is they don't work with other lower resistance gear, limiting the DPS they can do. Thanks for pointing it out.