Of course not, but plenty do when running a task where the user is unlikely to make inputs and also doesn’t want the machine to sleep. Firefox can call org.gnome.SessionManager.Inhibit over dbus with the “video-playing” description, same for VLC. Transmission can call that interface while a transfer is in progress (with a config toggle). It seems a pretty reasonable default for samba to do the same while a long-running file transfer is ongoing.
[Samba] doesn’t copy your files for you.
Sure but it has to know when a transfer is running. It would be nice to have the option to inhibit sleep if the transfer is runs for a significant amount of time.