The duckduckgo icons are labelled as such. Plenty of icon sets will have recognisable brand assets for the likes of social media platforms and so on, which will allow use according to certain guidelines.

Their inclusion probably isn't an issue in itself, but the blanket guidance in the copy above the result will be.

> The icons below can be used for Personal & Commercial purposes. They can be used on your website, branding and designs.

Obviously assets relating to existing brands will never be permitted for use in the branding of an unaffiliated party. onassar may want to update the guidelines to note such exclusions.