Agreed, this is a pretty misleading caveat. The documentation on embedding ACF in your own plugin states:
You can include the (free) Advanced Custom Fields plugin within a free or premium theme/plugin.
http://www.advancedcustomfields.com/resources/including-acf-in-a-plugin-theme/?version=4
But, what it leaves off is that if you do this your premium plugin will become open source.
