It's a gag about the police investigating a candy manufacturer that makes terrible candies. It's not necessarily supposed to have perfect internal logic! One of their candies has spring-loaded spikes that are supposed to pop out and stab the consumer! That's, like, not even remotely logical, and can't really be parsed logically.
I will guarantee you none of the troupe was sitting around saying "Okay, in this skit with the pepperpot saying she can't tell this butter substitute from a dead crab, how are we going to convince the audience that he both actually tasted the crab, and that crabs are normally used as a spread on your morning toast?"
No, they sat there and said "You know what would be funny? You know those commercials where the housewife can't tell the difference between real butter and margerine? How about we take that and make the other thing completely stupid, like a marmot or a hedgehog, or a dead crab or something?" To which the others said "You know, I think you've got something there!"
So while they were sitting around writing the Crunchy Frog sketch, I can guarantee you that none of them piped up and said "I know that 'after monosodium glutemate' line is pretty good, but we can't use it, since the character specifically stated they 'don't use any artificial ingredients'."
No, what happened is (probably) they were brainstorming things that could be put into candies, but shouldn't- sheep intenstines, pig swill, stainless steel bolts, bird vomit, etc.- and one of them came up with the line "covered in lark's vomit". The rest thought that was pretty good, but had to elicit a reaction from the constable, but a reaction that could just as quickly dismissed.
Hence, the "don't say nothin' about lark's vomit on the box" sequence, whereupon they could then move on to the other candies.