I agree, it leaks, but not every time and nearly as much as the $2000+ Knight 500 with its supplied Red Full Plastic Jacket ignition. That is the only way you can buy that rifle.
Also take into consideration, this video was made back in June 2018 or earlier with a prototype rifle. They've had 7 months to improve on the production rifles that we haven't seen yet. Sounds like they made several shots, and the flame channel in the breech plug could have been filling up and needed cleaning out?
The Remington Ultimate cannot shoot Blackhorn 209 in its supplied configuration, without voiding your warranty. If you change to Luke's Ignition System, you also void your warranty.
I don't like the VariFlame any more than anyone else, but that is what they have chosen. Cecil must have gave them quite the sales pitch, and they bought it?
With that said, hate to sound like a broken record, but Luke or Hank will likely have a module and plug fix for this rifle. Then it will really shine!