I interpret this: he did not give "any satisfactory description". He is not the kind of person who like to embellish a description, he problably just simply tell the facts of their interview and maybe a simply statement of his opinion on Bingley.

I agree with you that he deosn't like to speak about Bingley too much and once he said the facts he would not go further

On the contrary Sir William loves to embellish a description and of course, he enjoys this topic and would be problably more willing to talk about it. So in my opinion that is why Lady Lucas has more information, at least more of the kind Mrs. and the Miss Bennets looking for.

I don't know if you have read S&S, but I tend to interpret this passage you have post like this on Chapter 9 of S&S:

Sir John called on them as soon as the next interval of fair weather that morning allowed him to get out of doors; and Marianne's accident being related to him, he was eagerly asked whether he knew any gentleman of the name of Willoughby at Allenham.

"Willoughby!" cried Sir John; "what, is he in the country? That is good news, however; I will ride over to-morrow, and ask him to dinner on Thursday."

"You know him then," said Mrs. Dashwood. "Know him! to be sure I do. Why, he is down here every year."

"And what sort of a young man is he?" "As good a kind of fellow as ever lived, I assure you. A very decent shot, and there is not a bolder rider in England."

"And is that all you can say for him?" cried Marianne, indignantly. "But what are his manners on more intimate acquaintance? What his pursuits, his talents and genius?"

Sir John was rather puzzled.

"Upon my soul," said he, "I do not know much about him as to all that. But he is a pleasant, good humoured fellow, and has got the nicest little black bitch of a pointer I ever saw. Was she out with him to-day?"

But Marianne could no more satisfy him as to the colour of Mr. Willoughby's pointer than he could describe to her the shades of his mind.