Coincidence, I think not!
I re-re-re-re-reading Strong Poison, one of the best, most rounded Dorothy L. Sayers novels (Nine Tailors is good, but this one has a dramatic quality over and above that prime example of the puzzle detective novel). And reading a little more closely than usual, I suddenly found the Dowager Duchess' remark on page 24 significant:
... I have been reading one her books, really quite good and so well-written, and I didn't guess the murderer till page 200, rather clever, because I usually do it about page 15.
And true enough, about page 15 (13 in this edition), we get the scene where Philip Boyes is actually administered the poison. Quite clever. I wish I had a first edition: perhaps in that edition, the fatal dinner is first described on page 13.