IANAP, just someone with a lay understanding that's been following a lot of these topics for a while now.

As I understand it McCulloch's requirement that wavelengths fit within the cosmic horizon is an assumption, not a proven logical implication.

Additionally I think a lot of physicists are dismissive because they expect someone who desires to upend mainline theory to at least demonstrate fluency with the mathematics of mainline theory. McCulloch falls short of this.

All that said, I think he's genuine in his pursuit and I've really enjoyed reading his blog over the years.