“there’s nothing stopping TLD registry operators serving A records at the apexes of their TLD zones.”
Seven top-level domains answer at the bare apex, including .ai, .tk and .ws. Type ai in a browser and you get a retro page for the company that runs the TLD. ICANN strongly discourages this and bans it for new gTLDs. The country-code registries do it anyway, because ICANN has no stick.