Theobold was somewhat speechless when Mary had no shame. She was indirectly telling him in the face that she was going to copy their military weaponry.
He already anticipated that it would happen when he decided to give them the weapons, but did she really need to say it aloud?
Some things were usually better to leave out despite everyone knowing the answer, so Mary's response came as a shocker to Theobold.
"I do wish you the best of luck in copying it." In the end, Theobold faked a cough and could only wish them good luck in doing so.
Though inwardly, Theobold suspected that something must have occurred, so he kept that in mind to investigate further.
Mary's concealment might be pitch perfect, but it was too perfect, which caused him to believe the reasons should be something else. Something that they needed to discuss privately to prevent news from leaking out.