Zerotier is not the same as tailscale although both can be used to do the same, but under the hood both are fundamentally different, ZT is layer2 like switch, so it’s like an Ethernet meanwhile TS is built on top of wireguard and is layer3. ZT allows broadcast/multicast and has own protocol, TS don’t. I use both among others, and ZT since around 2019, I found it reliable in some cases in IoT world while TS had better throughput in usual applications.

Yeah, they're not direct replacements. I think both models have have their pros and cons. In fact I tried both around when covid shutdowns started (server being in the office, me at home), and liked zerotier better; it was faster, and a more generous free tier. But now tailscale has won out for a couple of reasons; the main one, it's simply less flaky for us on macOS, especially for devs working overseas. No idea why and maybe there's simple fixes (that don't involve repeated connections/disconnections, hopefully). The other, tailscale has a few extra things that are nicer and easy to use like identity-based ACLs, funnel/serve, magicDNS, ssh management, etc.