AWS charges by hour. Google by minute. If your peak traffic only last a few minutes, hourly billing is inflexible, simple as that.
(Disclosure: I work on GCP)
I'd bet they could have seen significant cost savings on AWS by migrating to lambda, and gotten continuous scaling to boot.
Of course, there are lots of other consideration that you should be making when going even partially serverless, and it could be that the NYT chose not to experiment with Lambda for other reasons. For instance, you're effectively tied to CloudWatch for logging and monitoring, which could be a deal-breaker. Much of the processing could be happening in the DB, which would make Lambda moot. It may simply be that their estimated usage of Lambda was too costly.