Or some candidates go through the easy route and privately fork an existing solution from GitHub which they heavily modify it to submit that as their own to go to the onsite interviews.

"Take home projects" are not enough to identify this so-called "talent" that you are searching for. Just ask all the candidates if they have any significant open source projects or patches sent to relevant high-profile projects and you have raised the barrier to entry to weed out hello-world, junior, mid type candidates.

So yes, You're crazy.