That's not why TS makes this recommendation. Rather: campaigns are almost universally dependent on GSuite anyways, Google has a fantastic security team and while Dropbox's has gotten better in the last few years it's still not Google, it's much easier to accidentally copy sensitive stuff into a Dropbox folder than a GDrive folder, Google Drive works especially well with Google's cloud document viewers, which is important for other reasons, and you might as well just have one cloud dependency, not several.