It's simply an archaic way of saying "liquid" as can be seen the term "pot-liquor" which also contains no alcohol. Sometimes archaic uses of a word get retained as technical jargon as in this case. To be more specific, "liquorem" meant liquid in Latin, "licor" meant liquid in Old French, and "likur" meant liquid in Middle English. The narrower meaning of being an alcoholic drink only started showing up around 1300 and was most often used to refer to wine and using "liquor" to mean "liquid" rather than specifically alcohol only became obsolete around 1700.

The older meaning of "liquor" to mean "liquid" in general survived in a few terms like the technical term for water used in brewing and doesn't have anything to do with tax laws.

The effect of tax laws on brewing IS fascinating however.

For example, in early modern England the way that tax laws were set up, you often paid less taxes if you mixed weaker and stronger ale together than if you sold moderate-strength ale (which is probably why the false legend that porter was originally a mixed drink came from). Later on in many cities an ale (unhopped beer) license was cheaper to get than a beer (hopped beer) license which lead to a lot ale brewers getting busted for putting hops in the ale.

Later on taxation shifted over to malt, which helped encourage people to use pale base malt (which they could squeeze more alcohol out of) and then use a small portion of specialty malts to flavor the beer. It also made using ingredients that weren't malt in beer illegal until the Free Mash Tun act of 1880.

Later on wartime rationing, the Great Depression and post-war austerity caused the strength of British beer to crash massively, and beer in the UK is still often quite a bit weaker than what was sold on the eve of WW I.