>What is "from each according to his ability, to each according to his needs" if not a criticism of merit?
It is not a criticism of merit, it is a criticism of the structuring of society on the basis of merit - i.e. meritocracy. Marx isn't the only one to notice thing; philosophers as diverse as Rawls and Habermas have recognized it too. Nevertheless, the conversation was always focused around the reward due to innate capabilities that differ from person to person. Even then, there are opposing points, which can't be reduced to slogans picked out here and there[0].
> Per Marcuse, our reality principle is the performance principle, which can be summarized as "to each according to his ability".
This is not the case. The performance principle specifically concerns the pursuit of profit and growth, despite having enough (non-surplus) labour and machinery in modern society to sustain life without repression (restriction of the libido). It does not make any comment on a possible allocation of resources according to the principle that he who labours more deserves more. The whole point was that "to each according to his ability" is not how society functions - according to Marcuse, just as Marx, this fact is clearly not manifested in the existence of a class system in which some own MoP and others do not. The only way this could be conflated as a criticism of merit would be the view that the system he is criticizing rewards merit (and this component of it would not exist under a future system). The criticism of a wage system rewarding as if "merit" is being counted neatly fits into my (1) point.
>unlike today's leftists, he's not afraid to admit that (1) capitalism was necessary to get this far
I'm curious as to who (speaking in terms of "leftist" philosophers) actually denies this, "leftist" or not. Simply saying that capitalism has had and can have disastrous effects does not at all entail the denial of its necessity in history, or what it has produced.
>(2) to attack the idea of rewarding people based on their performance head-on
If this is true (which, owing to the "communist labour-voucher debate" it may not be), it's curious that Scott didn't address any of this critique. I could have read more, and Scott could have read more. Anyway, here's a secondary source on Marcuse's support for meritocracy, at least in education[1].
Even if what you said were entirely true, nowhere would it imply a criticism of the generic "merit" of the qualified surgeon versus a street sweeper when you need an operation, which is the strawman Scott was attacking. No amount of Marx or Marcuse will get that concept validated by Scott's criticism.
[0] "Right, by its very nature, can consist only in the application of an equal standard; but unequal individuals (and they would not be different individuals if they were not unequal) are measurable only by an equal standard insofar as they are brought under an equal point of view, are taken from one definite side only – for instance, in the present case, are regarded only as workers and nothing more is seen in them, everything else being ignored. Further, one worker is married, another is not; one has more children than another, and so on and so forth." (CotGP)
[1] "However, it seems equally obvious that Marcuse’s notion of intellectual-elite education would also serve to adjust and confine the limits of free choice, thought, and experience. Exhibiting an uncommon disrespect for human diversity, Marcuse justifies his elitism on the ground that his university system would select “from the school and college population as a whole, a selection solely according to merit, that is to say, according to the inclination and ability for theoretical thought.” In a word, Marcuse wholly accepts the basic assumptions and premises of meritocracy. He also has a quick, if not readily attainable, answer for the query, “What knowledge is of most worth?” Beyond Aristotle and heading toward a revival of Plato, Marcuse reveres a contemplative wisdom which would border the power of the divine" (https://www.marcuse.org/herbert/booksabout/70s/DeVitis1974Ma...)