Also, Lambda is just the glue code. You have to use many other AWS stuff that really locks you in deep, even if you functionallity could "theoretically" be ported to different providers.

If there is a real need to port your infrastructure you can do it in small steps. Using single cloud provider is also not smart. If I need to do huge data processing I will never do it with AWS although this is where the rest of the infrastructure is as Google has better solutions. Just pick whatever is best.