You are correct - sus can be their or your (formal) depending on the situation.
Because it's not likely that someone else would be introducing the parents of the person you are talking to. That would be a strange situation...
I don't know if the translation "They introduce me to your parents" itself is wrong. Isn't it a case of having to be there in order to see who is doing the introductions? I do agree with you, however, that "They introduce me to their parents" is the likeliest scenario.
Wouldn't it be correct to say "They are introducing me to their parents" as well? I understand it doesn't use the present progressive form (estar + gerund) but I thought any present tense sentence could be used in both ways. For example, "Voy" could mean "I go" as well as "I am going."
For "They introduce their parents to me," would it be Ellas/Ellos les presentan a sus padres a mí?
´´they introduce me to YOUR parents´´ sería ´´Ellas me presentan a TUS padres´´