Because not all public land (meaning government-owned) is a public space (meaning available for use to the general public, like a park or community room at the library).
Our taxes pay for military bases too. Those are also government land. I’m sure you wouldn’t try wandering in there for a nice walk or a picnic.
Slash it from what? They didn’t even publish a platform in 2020. They just re-published the one from 2016 (that criticized the “current president”, which was actually Trump at that time), and released a statement giving “enthusiastic support” to Trump’s positions. There’s no platform to slash.
https://www.nytimes.com/2020/08/25/us/politics/republicans-platform.html