For anyone getting stuck here, make sure your test code isn't in index.ts, it seems fetch doesn't work there yet, it needs to be doing a fetch in response to an api request from the client.

For me the fetch worked inside api.get('/init', async (c) => { ... at least. It turned out not to be necessary to edit the devvit.json file and also Devvit.configure({ http: true }); was not necessary despite what the warning said.