Not really. They are designed to be fast enough and even then only as a secondary priority.

> You can do 6 billion … hashes/second on [commodity hardware] … there’s only 4 billion ipv4 addresses. So you can brute force the entire range

This is harder if you use a salt not known to the attacker. Per-entry salts can help even more, though that isn't relevant to IPv4 addresses in a web/app analytics context because after the attempt at anonymisation you want to still be able to tell that two addresses were the same.

> And that’s true even if they used something secure like SHA-256 instead of broken MD5

Relying purely on the computation complexity of one hash operation, even one not yet broken, is not safe given how easy temporary access to mass CPU/GPU power is these days. This can be mitigated somewhat by running many rounds of the hash with a non-global salt – which is what good key derivation processes do for instance. Of course you need to increase the number of rounds over time to keep up with the rate of growth in processing availability, to keep undoing your hash more hassle than it is worth.

But yeah, a single unsalted hash (or a hash with a salt the attacker knows) on IP address is not going to stop anyone who wants to work out what that address is.