That sounds logical at first blush, it's a dramatically oversimplified (and thus, effectively wrong) explanation of what's happening.
The United States (consisting of the federal government and states) is a single sovereign entity. The constitution is a creation of, and subservient to, that sovereign entity- it is a self-imposed restriction on the exercise of that sovereign power and a description of the split of that sovereignty.
The 9th and 10th Amendments are therefore by definition meaningless. Of course all powers that are not granted to the federal government flow to the states; there is nowhere else for them to possibly flow. They are simply restatements of what it means to have a federated government.
Of course, the issue is even more complicated than THAT; because ultimately the federal judiciary (specifically the Supreme Court) is the embodiment of the constitution as a document and the ultimate arbiter as to its meaning; in that respect, the split of sovereignty is not so much defined in the constitution as it is described in broad strokes in the constitution and then assigned to the Supreme Court for specification. In that respect, the issue of the 'federal government usurping those powers' is a meaningless statement in the broad scheme; it cannot by definition usurp what it is legally entitled to, and the Supreme Court defines what it is legally entitled to. The only way the federal government could usurp powers it was not legally entitled to would be to disobey a ruling of the Supreme Court that an exercise of its power was ultra vires