That is effectively how sub-agents are implemented at least conceptually, and yes, if you build your own coding agent, you can trivially implement sub-agents by having your coding agent recursively spawn itself.

Claude Code and others have some extras, such as the ability for the main agent to put them in the background, spawn them in parallel, and use tool calls to check on the status of them (so basic job control), but "poor mans sub-agents" only requires the ability for the coding agent to run an executable the equivalent of e.g. "claude --print <someprompt" (the --print option is real, and enables headless use; in practise you'd also want --stream-json, set allowed tools, and specify a conversation id so you can resume the sub-agents conversation).

And calling it all "summarising" understates it. It is delegation, and a large part of the value of delegation in a software system is abstraction and information hiding. The party that does the delegation does not need to care about all of the inner detail of the delegated task.

The value is not the summary. The value is the work done that the summary describes without unnecessary detail.