What central idea is developed by Mrs. Higgins's
few lines in this scene?

By speaking at the same time, Higgins and Pickering  show they are no more sophisticated than Eliza.

Explanation:

In the drawing room scene in Pygmalion Professor Higgins presented Eliza to his mother, who was not pleased with the visit as Higgins tended to act objectionably in front of her guests, to test the progress of his experimented.

Eliza, having already being coached on what subjects she can safely converse in, does so beautifully but comically, talking correctly while using slang.

During the course of the evening Higgins and colonel Pickering talked over each other, a fact which made them as unsophisticated as their experimental subject, as Mrs. Higgins pointed out to the two men.

This is even further emphasized in another scene where Higgins had to be reminded to mind his manners in front of Eliza, and at the end of the play she even has better manners than he does.

B. By speaking at the same time, Higgins and Pickering show they are no more sophisticated than Eliza.