Of course it isn't. It could reload the config from the same path it loaded the config from in the first place. Like practically all other software has done for decades.
The source of a config doesn't necessarily need to be from a config file. Config loading is abstracted. So it requires input, and signals provide no way to pass arguments, so it's not workable. See https://github.com/caddyserver/caddy/issues/3967
This sounds like a design decision you've made, not an inherent limitation. You can read config from files, like practically all other software has done for decades.
Of course it isn't. It could reload the config from the same path it loaded the config from in the first place. Like practically all other software has done for decades.