Hi sdfarquhar,
Thanks for reaching out to us on Microsoft Q&A forum.
You are correct that adding users from Unit A to a team in Unit C will give them access to the data that the team has permissions to in Unit C, which could include other tables besides TableX.
But if we go by the conditions given in this particular question the answer given is the right one.
It is at a beginner level that this module aims to give information.
We can always create and assign Security Roles depending on the use cases of the customers.
Hope we were able to answer to your query.
If you are still facing any issue, please let us know in the comments. We are glad to help you.
If the information is helpful, please Accept Answer so that it would be helpful to community members.
Thank you.