They are right. Google shows the synonyms for the keywords being searched. These synonyms are known as LSI (Latent Semantic Indexing) keywords. In simple terms, these are the keywords that are semantically related to your primary keyword. It is based on user search patterns and behavior, how one keyword search is usually linked to another keyword search.
Here, search query is buy genesis framework. However, Google highlighted (highlighted in red color) LSI keywords found:
1. buy - purchase
2. genesis/genesis framework - StudioPress (as this is StudioPress’s product)
Hope it is clear now
Tip: In case of keyword optimization in your article, avoid keyword stuffing. Use LSI keywords for the same purpose. Even Google has pointed that by using more such keywords, your web page typically ranks better.