Interestingly, Frances, the "best of the best" and "excellent" language was recently edited out of the Medium Distribution Guidelines for exactly the reasons you mention. https://help.medium.com/hc/en-us/articles/360006362473-Medium-s-Distribution-Guidelines-How-curators-review-stories-for-Boost-General-and-Network-Distribution

You also might note that this document used to be called Medium's Quality Guidelines but now it's Medium's Distribution Guidelines... Because you're absolutely right that "We will Boost what we find suitable" is the most accurate.