I'm not familiar with Redisearch either but I agree, there's definitely a bent to this in that they made something that isn't ES, then compare it to ES via a a benchmark that shows how poorly ES performs at...being something other than ES.
The impression I got was that they were trying to demonstrate for two specific workloads, how much more a single node of RS can do than a single node of ES, and that we should extrapolate the savings and performance if scaled out from there.
A properly deployed ES cluster versus a single RS node isn't a fair comparison either. It's a strained comparison in any case.