Safari seems to ignore my DNS Server
Safari is not doing anything untoward here. My DNS based block for reddit.com worked fine - but not www.reddit.com, old.reddit.com (even though the Eero block includes subdomains), and so on. From the command line:
$ nslookup www.reddit.com
Server:     192.168.4.1
Address:    192.168.4.1#53

Non-authoritative answer:
Name:   www.reddit.com
Address: 192.168.4.1

Because of that, I expected Safari to resolve www.reddit.com to 192.168.4.1. However, www.reddit.com (and others like old.reddit.com) are actually CNAMEs:
$ dig -t cname www.reddit.com
... snipped output ...
;; ANSWER SECTION:
www.reddit.com.     300 IN  CNAME   reddit.map.fastly.net.

So, it looks like Safari looks up www.reddit.com, sees that it is a CNAME for reddit.map.fastly.net, and then resolves reddit.map.fastly.net and is able to load the page - while displaying www.reddit.com in the URL bar.
I am not saying Safari is doing anything wrong here - this seems like totally reasonable behavior - it was just unexpected to me.
I have now added a DNS block for reddit.map.fastly.net, and everything works as expected.