Here's how I talk about it in the FAQ that Nominators are trained with:

Can I only nominate stories from my publication for Boost?

You can nominate any story for Boost, as long as it's related to your areas of expertise. While nominating stories from your publication gives you the greatest benefit (we've seen some publishers use their Boost nominator status as a way to recruit more submissions to their publication), the goal with the program is to better utilize the topical expertise of publishers to get better content seen across Medium. As always, good karma bonus points for nominating new writers!