that looks more like TCL than mIRC
It is what we call the "IRC protocol" - the format of the data that is actually being sent over the network between mIRC and the IRC server. The given command is what mIRC produces if you type "somedata" on #somechannel, or when you type "/msg #somechannel somedata". It is indeed sent to the channel, and hence delivered to either all users on the channel, or (on error) none at all (*). More details are in the original
IRC RFC (1459).
(*) modulo exceptions such as netsplits