Bear in mind this article was written back in March 2017; quite a few things have changed/improved in GAE-land since. To name a few, off the top of my head:

- Java 8 is already generally available on GAE standard and, on top of that, previous limitations wr/t class whitelisting, threading capabilities and others have been lifted

- appcfg is pretty much outdated these days; all GAE management operations have been progressively integrated into gcloud app subcommand, which also fully supports newer GAE features like Flex, firewall rules, etc.

- GAE standard instances are indeed limited in computing power, but it should be enough for most web-based applications. Again, GAE Flex fills in the gap here in case more muscle is needed, since it leverages GCE-class instances

- Besides the already mentioned background work options (background threads, task queues, cron jobs), there's also Pub/Sub which can be used to inter-communicate GAE services or even other GCP services like GCE or GKE