As in the case of every great political edifice, the materials composing the American system are derived from many different sources, and are characterized by unequal values, both as to endurance and as to latent possibilities. The appearance of definiteness and finality which it derives from its embodiment in specific constitutional documents and other authoritative words is to a large extent illusory. Its real origin and meaning are very much more doubtful and complex than these words intimate. Historians are no more agreed as to the former than political theorists are to the latter.
So what do we take from this? The Constitution, well that's not important. It cannot be, if its authority is largely illusory. Additionally, the rise of the professional (progressive) historian brought disagreement, because they too didn't see anything worthy in the Constitution. This benefits political authors and journalists who also have a similar mindset, because now they don't have to point to a friend they work with, they can point to some "distant" "expert" who by only a surface-level examination appears to be unbiased. So wink wink, nod nod over here, wink wink nod nod over there, everybody is in agreement - the Constitution sucks. These are old, outmoded ideals and we should progress toward something which is clearly better. We should progress toward something which is more concrete and not an illusion. Three pages in, Croly clarifies:
Emphatic, however, as was this assertion of its direct control over its own political institutions by the primitive American democracy, its willingness to restrict its own effective political power was no less definite and insistent. It did not show the slightest disposition to translate this supposedly effective popular control over the institutes of government into active popular control over governmental behavior. The democracy abdicated the continuing active exercise of effective power in the very act of affirming the reality of its own ultimate legal authority.
So you see, the illusion is the contradictory assertion of direct control, but yet a restriction on its own power. You need full total control, nothing less! Without full total control, that's the illusion. It's an abdication of effective power, that's what he's saying. The Constitution is, in his view, not important because it's a joke.
People with a mindset such as this cannot understand governmental limitations. Government is force, and these were the people, these progressives, who were born to be our masters. Government should be big, it should be unlimited, and of course! it should be the progressives who are in power until eternity.