You are of course free to believe that.
I no more know what an idealist's ideas are made of than you know what a materialist's quarks and bosons (the current "atom" level ala Democritus) are made of. What one chooses to believe at that level is one's of choice of ontology.
It doesn't matter that they choose to believe something, though. What matters is whether or not the thing they choose to believe has meaning.
Suppose an idealist and an old school materialist are sitting in a room, and you put a nail in front of both of them. The idealist sees an idea of a nail. The materialist sees a manifestation of a material nail.
But both of them are seeing the same nail. If you ask the idealist to hammer it into the wall, he grabs an ideal hammer and hammers in an idea into the wall. Ask the materialist, and he will grab a physical hammer and hammer in a physical nail into the wall. The materialist would likely pick up the same hammer that the idealist would pick up, and no matter who hammered in the nail, both would likely agree that the nail was hammered into the wall.
In both cases, the nails they
see are simply representations of the nail. In neither case is their representation of the nail the
actual nail. And in both cases they are able to interact with the same nail.
Before you figure out if the real nail is the idealist's image of the nail, or if the real nail is the materialists image of the nail, there's a bigger question you need to worry about; and that is this.
What does it mean
for one of them to be the real nail?