The point TechRadar were making was that it cached images in internal storage, so if you have a large gallery synced there, this could add to a lot of space.
I don't think rushmore drew attention to this as a criticism of Dropbox, but as a further reason why for some people the SD is not a substitute for internal storage. (Though if the app does cache a lot of info, it might be nice to be able to choose the location if there is a choice available).
I can't comment directly as I stopped updating the Dropbox app a long time ago. Ironically I in fact stopped when they introduced the photo syncing features that are the subject here, since to me this was unwelcome feature creep - of course it may have been a desirable addition for others.