With my standard disclaimer that nobody need care a single iota about my opinion, I would avoid this strategy on philosophical grounds.
This looks to me like a “silent dependency”. If you reorganize the structure of the form, that validator will break.
So IMHO, parent should be off-limits. Using it is an indicator that the validator is on the wrong component, and needs to be higher up. There is an example of what I would consider a much more robust way of handling this in the official Angular docs on cross-field validation. The validator should be on the group, not on one of the individual group members.