I still see this as flawed. Avoiding all vendor lock-in means making every possible piece of software you write completely generic - no GIS from Postgres, no third-party queue/messaging servers/services - none of it.
We're "locked in" to AWS - in that it would take a massive effort to move to Google/bare metal, despite not using any AWS-written servers (that is, we're FreeBSD, MySQL, Redis, etc.). But the benefits of AWS have been huge for us, and anyone saying they're outweighed by the potential downside has no idea what they're on about.
The right tool for the job is sometimes a vendor with specific services that, yes, sometimes have a lock-in component, even for long-term use.
In this specific case this particular BaaS implementation may not be full-cooked enough to entice you onto it, but saying vendor lock-in is bad as a blanket statement or incontrovertible truth is just silly.