You don't stop being the corresponding authors of a paper when you change companies, and whatever "unclear circumstances" you imagine took place when they left, they were also re-hired later, which a company would only do if they were in good standing.

In any case, those "Google contacts" also expressed concerns with how Cheng et al. were doing their study, which they ignored:

3.4 Cheng et al.’s Incorrect Claim of Validation by Google Engineers

Cheng et al. claimed that Google engineers confirmed its technical correctness, but this is untrue. Google engineers (who were not corresponding authors of the Nature paper) merely confirmed that they were able to train from scratch (i.e. no pre-training) on a single test case from the quick start guide in our open-source repository. The quick start guide is of course not a description of how to fully replicate the methodology described in our Nature paper, and is only intended as a first step to confirm that the needed software is installed, that the code has compiled, and that it can successfully run on a single simple test case (Ariane).

In fact, these Google engineers share our concerns and provided constructive feedback, which was not addressed. For example, prior to publication of Cheng et al., through written communication and in several meetings, they raised concerns about the study, including the use of drastically less compute, and failing to tune proxy cost weights to account for a drastically different technology node size.

The Acknowledgements section of Cheng et al. also lists the Nature corresponding authors and implies that they were consulted or even involved, but this is not the case. In fact, the corresponding authors only became aware of this paper after its publication.