Science is restricted to reality. If you abandon reality, truth, logic and science no longer apply.
"...time is a dimension, yet we do not thereby say what time is;
Time is identified by observation to be one of the dimensions of this universe. Just as length dimensions quantify spacial extent, time quantifies the extent, or length, of existence in this universe. It is a measure of length for real things. Hawking agrees with this.
" we cannot say any better what a dimension truly is."
It's precisely defined, just as length is.
"We only know that's an apple because we have a very good theory about apples."
There is no theory of apples. That a thing is an apple, comes by description that differentiates it from other things.
"When we accept Hawking's presupposition, we can calmly approach the opponent comparing how well each theory matches the universe."
I know what Hawking does, but have no clue what his presupposiiton is. The is science, so it is restricted to that. Handwaving to support claims and appealing to the present limits of knowledge and understanding to support handwaiving, does not amount to science.
This should be endlessly repeated to the wannabe Humean skeptics on these threads.