I think she is not admitting it publicly. Marquis only have a hunch and the way to investigate properly is to conquer them.
Its not just him who knows though seems like everyone does. I mean if she has the crystals or w/e they would obviously be from a mine?
basically she didn't want to reveal the crystals due to the taxes, however she knew if she didn't it would upset the emperor and the potential issues with that so she just sighed and reveals it anyway to avoid that^^ so yes everyone knows
p.s. scroll to the bottom of chapter 34 and read the the tl notes, they should help explain better :)
I see
So they were not supposed to know shes the supplier yet they all do its confusing?