You can get through to a web server, but that web server uses DNS records or those routes to hit other services necessary to render the page. So the server you hit will also time out eventually and return a 500
The issue here is that this outage was a result of all the routes into their data centers being cut off (seemingly from the inside). So knowing that one of the servers in there is at IP address "1.2.3.4" doesn't help, because no-one on the outside even knows how to send a packet to that server anymore.
Or when trying ips directly: https://www.lifewire.com/what-is-the-ip-address-of-facebook-...
I would have expected a DNS issue to not affect either of these.
I can understand the onionsite being down if facebook implemented it the way a thirdparty would (a proxy server accessing facebook.com) instead of actually having it integrated into its infrastructure as a first class citizen.