You cannot just skip the tax return.
You have to file the return, report the chinese pension, and claim treaty exemption.
Big difference.
As long as they have more than $4300 income, they cannot be LZ's dependent. Whether this income is taxed or not is difference issue.
In fact, there is no good reason to claim non-child as dependent anymore after Trump tax cut. The personal exemption is zero so there is no gain in taxes. Being an dependent is not eligible for the 2020 two rounds of covid relief.