Spotted by @squahtx.

Some users, e.g. appservice users, won't have any devices whatsoever. If we query their homeserver to retrieve that user's devices, it should respond with an empty list of devices. In the processing of that response, by the time we reach this line we have a devices list from a successful response whose size is at most 999.

We transform devices into a dictionary {d["device_id"]: d for d in devices}. Assuming there are no repeated device_ids, this is a reversible transformation.

Here, cached_devices is:

Suppose we have never cached devices for this user, and that the devices list is empty. Then cached_devices will be an empty dictionary and so too will the transformed version of devices. Thus we'll set ignore_devices = True and not cache the fact that this user has no devices.

Then, whenever someone wants this user's device again, we'll repeat the federation request again.