This assertion needs a lawsuit and deep pockets. Most startups will not be able to do this.

Infact the patent holder can file a lawsuit against a startup. Now even if the startup can prove that the patent is invalid, it has to spend precious money on the lawsuit.

Yes, we live in a world where patents exist. It's still possible to be correct about certain things in that world, and the fact that one does not like that patents exist doesn't entitle people to be incorrect about how the patent system works.