Well there is a very simple way to check this. We just need to go to the CFTC site and look for a document called "FCM Financial Data". It's both in PDF and Excel format and it lists the financial information of all of the clearing FCM's.
https://www.cftc.gov/MarketReports/financialfcmdata/index.htm. So if NinjaTrader is a clearing FCM, it should show up on this list. And as of May of 2021, NinjaTrader Clearing LLC is indeed a clearing FCM, what do you know?? It's one of the FCM's that has the lowest adjusted net capital but nevertheless it satisfied the $1 million capital requirement for clearing FCM from company funds only that's completely segregated from customer's trading funds. And this explains why I don't see Dorman or Philips listed as their associated brokers anymore on their website. But this is as of May of 2021. Now it's July 2021, are they still a clearing FCM? They should be considering that they have bought out TransAct Futures which would provide them the infrastructure that they need to be a clearing broker in futures. Wow, a lot has happened since I stopped using NinjaTrader.
But if they still are a clearing FCM, why are they so reluctant to answer your email inquiry to confirm this? They should be proud of advertising this fact.