I would say that broadly categorizing FaaS as vendor lock-in would be disingenuous on its own. Practical FaaS is a simple concept as far as developer implementation is concerned. Write functions, get HTTP context, and return a result. This article really only touches on problems with vendor implementation of FaaS, AWS may have these issues with I/O and lifetime of a function, but there are many open source implementations that you could install on custom hardware that bypasses these issues. Even some of the issues that seem like fundamental issues with FaaS (non-addressable instances, unable to use specialized hardware) are already solved by other open source self-hosted FaaS solutions.