Intelligent Designists (the “-er” is what the “-ists” posit…) do graps evolution. They even accept it, in general cases. Where they deviate from scientific thought is when they posit that an Intelligent Designer is necessary to explain the diversity of life.
I’ve often said that thast there are essentially two aspects to evolutionary theory: the mechanistic theories, and the historical pathways. Few IDists deny that the mechanisms function as adverstised; they simply deny that they can produce all known “end results”, because the specific historical pathway that is necessary to produce a given trait or structure via those mechanisms is either unknown or incomplete. For example, how can natural selection produce the vertebrate eye, or a bacterial flagellum?
In cases where science is unable to provide an answer at the time, the default assumption of IDists seems to be that the structure/behavior/other trait in question must therefore be too complicated for science to resolve the pathway, therefore a Designer must have been responsible. In other words, the Intelligent Designer is relegated to being an ad hoc explanation for unresolved historical pathways by its own proponents! ID becomes Special Creation, writ small. Eventually, science may catch up to a specific problem, and a suitable pathway is discovered (e.g., bacterial flagella may have evolved from primitive protein secretion structures), rendering the Designer unnecessary in that instance. And so it goes.
What IDists do fail to grasp, in my opinion, is the fact that evolutionary theory as currently formulated does not render an Intelligent Designer (or just plain God, if you prefer) non-existant, only that the explanations provided render such a designer unnecessary. A tinkerer is not required to explain life as we know it. That does not mean such a tinkerer does not / cannot exist.
The main point being that science is meant to be free of any particular metaphysical assumptions. God may or may not exist, but we have no way of knowing one way or the other. Therefore, it would behoove us to find explanations that do not require a god, just in case. ID, on the other hand, requires that God exist (and that God tinkered at one or more points in Life’s history, and that He left identifiable hallmarks of this tinkering…). On this basis alone, it is less powerful as an explanation, simply because it is absolutely wrong if there is no God.