I'm going to say "no", without any official source to back that up. However, Splunk is not a traditional "big data" product, doesn't store it's data in a standard format, and doesn't use SQL to query data. All of those things would seem to disqualify Redshift.
As I read it, Fivetran merely copies Splunk data into Redshift. I don't see where Splunk can use the Redshift data. Consider using the free trial to see how it works, but understand that Splunk probably will not support you.