Hi, I can confirm that close-chain mechanism can be simulated in Isaac Sim, but you have to modify the USD file such as excluding the un-actuated joints from articulation. I recently test the sim-to-real transfer, and the result is acceptable, so there should be no problem simulating close-chain (but you should increase the position/velocity solver iteration or decrease physics_dt to ensure a good sim-to-real performance).
You can also check this post: Support Parallel Robots?