Yes, the statement
Today I learnt how to wax a metaphor
is itself a metaphor.
Without more context, it's not possible to say for sure what the intended meaning of 'to wax' is in that metaphor, but the most likely meaning is
To increase [the metaphor] gradually in size, ..., strength, or intensity....
[wax. (n.d.) American Heritage® Dictionary of the English Language, Fifth Edition. (2011). Retrieved December 30 2015 from http://www.thefreedictionary.com/wax .]
Another possible meaning, that seems less likely to me, is
To coat, treat, or polish with wax.
In this latter sense, 'to wax' a metaphor would mean to make it more slippery, as with waxed skis, or more polished, as with floors.
The statement is a metaphor, but whether or not it can
... be read literally
does not bear directly on whether or not the statement is or is not a metaphor.
Other things than metaphors cannot be read literally. One prominent example of another thing that cannot be read literally is nonsense, which, if read metaphorically, is no longer nonsense.
Some metaphors can be read literally, but are nonetheless metaphors. This is by far the most common case for metaphors. An example is your idea that 'waxing' in the sense of 'recording' is literal. The actual referent of 'to wax' in that sense is 'to record in wax', which itself refers to the substance used for phonographic recording. Thus 'to wax' in the sense you took for literal means 'to record [on wax]'--but phonographic recordings long ago ceased to be made in wax:
A decade later, Edison developed a greatly improved phonograph that employed a hollow wax cylinder instead of a foil sheet. This proved to be both a better-sounding and far more useful device. The wax phonograph cylinder created the recorded sound market at the end of the 1880s and dominated it through the early years of the 20th century.
A metaphor is a metaphor because it is
a poetically or rhetorically ambitious use of words, a figurative as opposed to literal use.
(See Hills, David, "Metaphor", The Stanford Encyclopedia of Philosophy (Winter 2012 Edition), Edward N. Zalta (ed.), URL = http://plato.stanford.edu/archives/win2012/entries/metaphor/.)
A deceptively simple but nonetheless complete definition of metaphor is this from Silva Rhetoricae:
A comparison made by referring to one thing as another.
In the case of your example, a metaphor is referred to with a verb that applies to skis, or floors, or another thing entirely.