It's a mild tradeoff the post glossed over: if you're handling IPv6 addresses with a zone specifier, the first use of a particular zone specifier will cause an allocation to intern that string.

Thankfully, the vast majority of IP-wielding programs never touch zones, and IPs sans zone specifiers are always allocation-free. And even for the long tail of zone-aware programs (e.g. corerad, a router advertisement daemon), you end up with one allocation per unique zone string, not one per IP value, which generally amortizes to alloc-free in steady state.