I still don't see how this is radlib. It might be radical to your or me, but how is it a radical form of liberalism instead of just a form of mainstream liberalism?

We should be careful about watering down words. The Atlantic being radlib would mean there's little liberalism that isn't radlib.

How is The Atlantic radlib? There's not really anything radical about it.

Is he really making a mistake? It seems to me like he's engaging in immanent critique of The Atlantic.

He's showing how what it does contradicts and differs from what it says it does.

What do you mean by the Marxist conception? Marx himself sometimes uses the term middle class.

Here's a few examples.

The Communist Manifesto, Chapter 1:

The lower middle class, the small manufacturer, the shopkeeper, the artisan, the peasant, all these fight against the bourgeoisie, to save from extinction their existence as fractions of the middle class. They are therefore not revolutionary, but conservative. Nay more, they are reactionary, for they try to roll back the wheel of history. If by chance, they are revolutionary, they are only so in view of their impending transfer into the proletariat; they thus defend not their present, but their future interests, they desert their own standpoint to place themselves at that of the proletariat.

The 18th Brumaire of Louis Bonaparte, Chapter 1:

The bourgeois monarchy of Louis Philippe can be followed only by a bourgeois republic; that is to say, whereas a limited section of the bourgeoisie ruled in the name of the king, the whole of the bourgeoisie will now rule in the name of the people. The demands of the Paris proletariat are utopian nonsense, to which an end must be put. To this declaration of the Constituent National Assembly the Paris proletariat replied with the June insurrection, the most colossal event in the history of European civil wars. The bourgeois republic triumphed. On its side stood the aristocracy of finance, the industrial bourgeoisie, the middle class, the petty bourgeois, the army, the lumpen proletariat organized as the Mobile Guard, the intellectual lights, the clergy, and the rural population. On the side of the Paris proletariat stood none but itself.

Capital Volume 1, Chapter 25, Section 4:

Pauperism is the hospital of the active labour-army and the dead weight of the industrial reserve army. Its production is included in that of the relative surplus population, its necessity in theirs; along with the surplus population, pauperism forms a condition of capitalist production, and of the capitalist development of wealth. It enters into the faux frais of capitalist production; but capital knows how to throw these, for the most part, from its own shoulders on to those of the working class and the lower middle class.

There is no place yet in America for a third party, I believe. The divergence of interests even in the same class group is so great in that tremendous area that wholly different groups and interests are represented in each of the two big parties, depending on the locality, and almost each particular section of the possessing class has its representatives in each of the two parties to a very large degree, though today big industry forms the core of the Republicans on the whole, just as the big landowners of the South form that of the Democrats. The apparent haphazardness of this jumbling together is what provides the splendid soil for the corruption and the plundering of the government that flourish there so beautifully. Only when the land — the public lands — is completely in the hands of the speculators, and settlement on the land thus becomes more and more difficult or falls prey to gouging — only then, I think, will the time come, with peaceful development, for a third party. Land is the basis of speculation, and the American speculative mania and speculative opportunity are the chief levers that hold the native-born worker in bondage to the bourgeoisie. Only when there is a generation of native-born workers that cannot expect anything from speculation any more will we have a solid foothold in America. But, of course, who can count on peaceful development in America! There are economic jumps over there, like the political ones in France — to be sure, they produce the same momentary retrogressions.

From Friedrich Engels's 6th of January 1892 letter to Friedrich Adolph Sorge