Good point. But something has to be setting those MAC addresses...
And it looks like, curiously, explicitly configured interfaces have their setup expressed as .link units even if networkd is not in use[1]. A comment[2] states: “.link units are honored by udev, no matter if systemd-networkd is enabled or not”.
It seems that .link units are nowadays interpreted not by networkd (which NixOS gates with useNetworkd) but by udevd (which it does not). The documentation for them (but not for udevd) even points that out[3] if you’re the kind of person who reads introductions: “link.link: A plain ini-style text file that encodes configuration for matching network devices, used by systemd-udevd(8) and in particular its net_setup_link builtin”.
Given that OP uses dhcpcd, it's unlikely to be systemd-networkd.