...I think it's clear from that first OED definition that this was not a malapropism. However, you've caused me to take a second look, and realize that I was so predisposed to see a malapropism in that instance that I missed what is, right there on the surface, a bit of brilliant ironic wordplay of an entirely different "species" by Sheridan's, to wit:

The THIRD definition of "specious" in the OED reads as follows:

"Superficially genuine or correct but in reality wrong or false...misleadingly sound or convincing."

That is the sense in which I have seen, and used, the word specious , and when you think about it, that's a pretty good definition of a malapropism! In order for a malapropism to be funny, in addition to other requirements, there must be a superficial appearance of correctness, a recognizable resemblance to the correct word that should have been used.