The ambiguity doesn't lie in they, it lies in the way the writer constructed that sentence, as the person you responded to already stated.

The writer (and the person they are communicating with) knows the plurality of the "who", an outside observer (us, the readers) aren't privy to that information. Clarification on the part of the writer would provide that context. But the sentence isn't written to be read to a 3rd party, but the other party (the person the writer is communicating with).

99.99% of people understand this intuitively, but this is the way you'd parse the understanding of that sentence.

And if you'll note, in my second sentence, "they" is understood to be singular—the writer.

E: and for Shits n' giggles: if neither party (the writer nor the person being communicated to) knows the plurality of the "who" they are referring to, then it's irrelevant information. They will discover who wrote it when they go searching.

And if you'll note, in that previous sentence, it's understood that I am using the plural they (the writer and the person being communicated to) in both uses of the last sentence.