Context. I know, it's annoying to always hear that response, but it's true.
For example, if you're having a conversation with a good friend and say this, they'll take it to mean "they", because you've presumably been having a conversation using du instead of Sie. Also, you must have just mentioned a group of people in order for "they" to be comprehensible.
On the other hand, if a salesperson is telling you about the benefits of their loyalty program, they will likely be addressing you in the polite form throughout the conversation (Sie), and clearly their attention is on demonstrating that you could save money.
Yes, there are situations where it could be ambiguous, but then you can anticipate that and reword it, use body language, etc.