> I asked him to remotely reboot the modem because there was high packet loss.

FWIW, as a customer (assuming residential), you can login to the website and check for area outages/impairments at https://www.xfinity.com/support/status-map. You can also use the Xfinity app to remotely reboot your cable modem, run diagnostics/check for outages, etc. See https://www.xfinity.com/support/articles/check-service-outage

> Both times I've talked with him, he noted the high packet loss, started to reboot the modem, and then asked me point-blank if we had any PoE switches on our network.

High packet loss typically suggests an RF impairment of some type. I don’t know how to explain the PoE comment but am happy to look at your connection if you want to email me off-list.  

> I said "it's up and working fine, why would I reboot it?".

In some cases a reboot will trigger a pull of the latest firmware, which might include security fixes, performance improvements, and other changes.

Jason