The i18n package's readme currently says:

Note that you will not need to specify domain for the strings.

This might be true if you do the string extraction yourself, but it sets false expectations for developers trying to use the package.

I suggest replacing this paragraph with an information that the text domain should match the plugin slug.

I've written a lengthy blog post about the importance of the text domain in WordPress internationalization here: https://pascalbirchler.com/text-domain-wordpress-internationalization/

Also, there's this part:

This is useful to trick WordPress.org translation strings discovery since at the moment, WordPress.org is not capable of parsing strings directly from JavaScript files.

Which is totally out of date now. See https://make.wordpress.org/core/2018/11/09/new-javascript-i18n-support-in-wordpress/