Taxes are a subject of frequent liberal conspiracy theories. You will see all sorts of blatantly false claims like this because left wing misinformation spreaders like Robert Reich make up their own tax calculations that have no relation whatsoever to actual tax law.
No need to limit this to "liberal" conspiracy theories. Trump and his admin's statements on how tariffs and other taxes work and who pays them have been full of blatantly false claims.
It’s a fair retort here, though, where the grandparent comment was clearly trying to grandstand in opposition to his perceived enemy tribe, mostly unprovoked.
Edit: in other words, it’s a fair interpretation of the comment to be saying “We wouldn’t have to deal with all this misinformation about taxes if there wasn’t some giant liberal conspiracy”, given that they weren’t replying to any specific part of the parent post.
Well, no, that is not a reasonable interpretation at all. For one, the commenter did not proclaim existence of conspiracies, but the existence of conspiracy theories. People mix these up a lot. Secondly, the other interpretation you propose exhibits roughly the same form as "X does A", so it's worth repeating that it does not mean "only X does A" either!
