Interesting question. I don't think that's incorrect, but it sort of depends on what you mean by "crouton."

The Oxford Russian Dictionary equates "гренок" with "crouton," and defines "сухарь" as "rusk" (often called melba toast in the US). The Большой толковый словарь defines "гренки" as "поджаренные ломтики хлеба" (toasted slices of bread), and "сухарь" as "высушенный кусок хлеба" (a dried piece of bread), and also a type of cookie/biscuit or confectionary product of the same form. Neither source lists a more specific definition for the diminutive form. Not the most helpful.

I believe that сухарики are intended to be eaten by themselves as a snack, rather than e.g. added to a salad like you would with American-style croutons, hence the equation with rusk (I could be off on this, though I've purchased them in Russian vending machines). You can eat croutons as a snack, of course, but they are conventionally supposed to go in salads or soups. Сухарики certainly do bear a close resemblance to American-style croutons in form, though, quite possibly more so than what would be called a "гренок" in Russia, which may be e.g. larger.

This may be a case where the concepts just don't line up exactly, so there's some flexibility.