Glad to hear things worked out for your client. The key thing is, YOU had to create an exit strategy just for your client.

If the specifications for the serverless platform were "open" then the client could move their stack to different vendor with insignificant planning/risk.

An analogy would be vendors somehow changing machine virtualization such that your code was required to be very hypervisor-aware to function, and porting from vmware to xen imposed significant cost and risk.