Cloudflare Workers isn’t a Node.js environment, so unless you are packaging it with something like webpack the import won’t work. I presume that is the issue here.
Unless there is a specific issue with that package which only the team knows (cc @KentonVarda@signalnerve) or someone else, the fact that some packages work makes this issue not really a Worker’s one so this may not be the best place (Stack Overflow, maybe?).
Also, have you actually tried putting it in exactly as they say const Joi = require('@hapi/joi');? It shouldn’t matter, but you might never know.
Two things I can recommend there:
open an issue in the wrangler CLI GitHub report so that Ashley (who I believe follows the development of that can read it and check on it)
maybe @cloonan or @sklabnik can ping Ashley and let her see this conversation and maybe make her open a Community account I could try to ping her on Twitter, but they are the best bet since I don’t know how much she will reply to a random tweet even if we met once, maybe I’ll try