Conceptually it’s the same thing - there was even an Admin interface for installing extensions at one point. Packages can be installed either via composer or through a “manual installation” which is just like extracting a zip in the right directory. The considerations are similar to Wordpress too, which is to say that if you install an extension but modified the themes then it’s possible the extension won’t present correctly and will need to be fixed.
That’s what the developer is likely accounting for… you install the module, test it… realize one of your changes has surpassed something or there’s a latent incompatibility and you have to address it first. In a Magento world, we typically do things locally, then to a series of integration and staging environments before finally deploying to production whereas with Wordpress folks often make their changes on the live instance.