I'm taking the parts of this write-up I don't have expertise with a grain of salt after seeig this.
Kessler cascades are real. Particularly at high altitudes. They're less of a problem in LEO. And in no case can they "[cripple] our access to space." (At current technology levels. To cripple access to space you need to vaporise material fractions of the Earth's crust into orbit.)
Well, maybe "higher", but not really high.
The lower the altitude, the larger the odds of making one, in a quadratic fashion. But also the lower the altitude, the less time it will last.
There is some space where it lasts basically forever but is small enough for it to happen. It's higher than LEO, and way lower than things like GEO.