I've been researching and I don't see a way to use secret chats through userbots.
I have seen that there is another library (telethon-secret-chat) that seems to do the job but I am surprised that such a good library like Telethon did not have mention of these chats in the documentation nor that nobody has asked about it in stackoverflow either.
Does Telethon support secret chats natively?
Telethon v1 as a library contains what is needed to interact with Telegram's API, and offers some abstractions to make it nicer to use.
Secret chats are not in-scope for the project, but it's definitely possible to use Telethon to build what's needed to use secret chats.
I (the main developer) simply have no interest in maintaining such a feature or even spending the time figuring out how they work.