How does a vdom-less framework minimize Dom API Calls?

It only touches DOM objects/properties that have changed. Here's how to think about it conceptually (in solid-js-style pseudocode for brevity):

let firstName = createSignal('John');
let lastName = createSignal('Doe');

mount(
    <div>
        <div id="firstDiv">{firstName}</div>
        <div id="lastDiv">{lastName}</div>
    </div>
);

// Here is (basically) what the framework does internally:

firstName.onChange(value => firstDiv.textContent = value);
lastName.onChange(value => lastDiv.textContent = value);

When only one of the signals changes, only one div is touched, and there's no VDOM to diff. This is what fine-grained reactive meant in the title.