That bit seems to be the missing in the docs.
I'd say that's a bare minimum requirement to start playing with these services. You need to get data in/out and with no socket support HTTP is the only way. If you can't get data in/out there's little you can create on these.
Then you have things such as log access etc etc for prod workloads.
Fastly C@E with Lucet is really promising but Fastly are lacking marketing/delivery/execution. You can't use it unless you are on the private beta which you can't access as no seats left and even then there's no pricing information and Fastly's current pricing model makes it a non starter for dev with $50 minimum fee instead of pay as you go.
Five years some form of WASI will be available on all the FaaS services by the big cloud providers. Good to see these services starting to appear.