You certainly do use the word Dropbox – with no other branding – in your naming of the product. You also use the Dropbox logo. Backup and Restore are generic words. Dropbox is a trademark owned by someone else – not you. Other WordPress backup apps use their own branding while providing a Dropbox backup option.
Your statement is more a matter of your perspective than of fact, clouded by your apparent motivation. In my opinion, you are deceiving users into a false belief that the product is by Dropbox.
1. You use the dropbox logo front and center to identify the product.
2. You use no other logo to uniquely identify the product as yours.
3. You use the word dropbox in the product URL.
You’ve seemingly had a good thing going here. The review is here for others to judge, and the real Dropbox is now aware of what you are doing.
It is my hope that the operators of the WordPress site have their own process to review and take actions regarding products such as this that may be deceiving customers by co-opting another business’s brand. This behavior undermines that of those who play by the rules of ethics and integrity.