You're not wrong. Maybe the dev means that once you download it, you can play it without being connected to Steam and the Internet? Some games are like that or something.

You need to connect to Steam to download the game. There's no getting around that. The game files can't appear out of thin air. They need to be downloaded from a server.

But once you have the files they are yours. You don't need to run the game through Steam (or even have Steam installed). Naturally, if you don't run it through Steam you won't get achievements or cloud sync.

It's no different from eg. GoG. You need to authenticate with GoG to download the game but once you have it it's yours.

Steam does provide DRM services and some games do use them (meaning you have to play the games through Steam). But developers are free to choose whether they want to implement them. I chose not to since I believe DRM only harms paying customers. Customers should be free to backup their games if they want to.