Looks like there is going to be a shift to using nftables in arch. The iptables package in core is currently for the legacy interface with an iptables-nft package for the new interface, but the core-testing iptables package is for nft interface and there is now a iptables-legacy package in core-testing.
My guess is they are moving packages that can work with nftables to depend on that instead of iptables which looks like it is shortly going to be using the new nftables interface anyway. Probably as part of migrating to nftables by default. Looks like docker does have experimental support for nftables in version 1.29 and that is when the dependency was added to the PKGBUILD script.
It does not look like nftables or iptables conflict with each other at a package level. And nftables can work with iptables rules.
It is probably worth just migrating to nftables now if you rely on managing iptables yourself.