I haven't heard that expression used for ages. Over 60 years, to be a little more exact. I wasn't sure then - and I'm not sure even now - quite whether that wording is what the man meant to say or whether he really meant 'as sure as eggs is eggs'. My considered opinion, refined over these past 60+ years, is that he was trying to be clever and paraphrase the well-known saying. It would have been in character, or as near as I can tell, my pre-teenage self not being into character reading to any great extent. (I'm not saying that I'm pre-teenage now, you understand, but that I was when I heard those words.)
It was at school, back in the days when the teachers (we called them masters in those days) always wore jackets, ties and GOWNS, although mortar boards had been put aside. This particular gentleman was the deputy headmaster and was standing in for our regular master (I have a feeling it might have been a history less) who was sick or attending a funeral or something. Anyway, this deputy headmaster, by name of Parsons, was Welsh. I'm not sure that his place of birth really has any bearing on the matter, but then again, it might.
Because Mr P was unfamiliar with the names of the boys in my form (we weren't called students back then) he had fallen into the habit of making up names. Well, not so much making up names, as using objects as names.
"You, Tomato," he would say, pointing to a pupil. (Just an example, you understand.)
One of the pupils had what turned out to be the unfortunate (or fortunate, depending on your sense of humour) name of Gammon.
You, there, Bacon. What's your name?" demanded Mr P.
He took some convincing that the answer was not a leg-pull and threatened the whole class with a visit to the headmaster. "I'll have you trotting along as sure as a couple of eggs!" he exclaimed.
I've never heard the phrase since, and I can find no trace of it on Google so I think he must have been making it up.