When someone says "No taxation without representation!" they don't mean "As a matter of fact, no one ever gets taxed by a government they don't have the power to vote for or against". (If that were true, there'd be no need to demand it.)

They mean "I would like our government to stop taxing people who don't get to vote for it" or "It is unjust for a government to tax people who don't get to vote for it" or something of that sort.

The fact that things aren't already the way you want them to be doesn't make it absurd to demand that they change to be that way.

(You might argument that governments don't give a damn what anyone demands and for that reason it is absurd to demand change. But I think that in fact governments do take notice of what people want, if they fear what the people might do if they don't get it. Whether that's voting them out of office or putting their heads on pikes or anything in between. And they will take more notice if more people are demanding whatever it is, and a large part of the point of saying things like "No taxation without representation!" is to get other people who aren't in the government to sympathize with your cause and maybe start demanding the same thing. So I think it's manifestly not absurd to make such demands, as such. Some particular demands -- "No taxation without $1M/year universal basic income!" -- would be absurd, but this one seems obviously not to be in that category.)