"You found an oddball use of the word recessive. ..."
I rather see it as a nifty new perspective on recessiveness.
The expression of a gene can be blocked by a number of things.
For critters with chromosomes in pairs there are a number of examples:
One is the presence elsewhere of a different gene that blocks it. Say, for instance, chromosome A has a dominant gene for purple polka dots (call it ppd).
Chromosome C might have a gene where the dominant form turns the ppd into white polka dots (wpd).
And chromosome Z might have a gene that undoes the effect of chromosome C
But, back to Chromosome A. At the same location as the ppd gene there can be, instead, a gene recessive to it producing green polka dots (gpd).
Now it gets to be fun.
There can be a gene recessive to gpd that produces yellow polka dots. But there can also be a gene dominant to the ppd one that produces no polka dots (npd)
These variations at one location are called alleles.
Probably every variant you can think of, plus some.
What this shows is simply that the same effects can occur if all the genes are strung along a single chromosome on a critter which has only one. Not oddball, it fits in perfectly.
And you are absolutely correct: it takes mutations to get the variants.
The word "recessive" is very unfortunate when applied to duplicated genes on a single chromosome. The word has a long established tie to sexual reproduction.
Gene duplication is itself a common kind of mutation. It is a source of variation.
Southack's argument is equivalent to arguing that A * A * A is really the same as A * A if you forget to perform one of the operations. I mean, everything is the same as everything else if you deny that transformations are significant.