It's around the 10:00 mark
"We are also doing tons of internal playtesting on the revised monsters.... along with the new encounter building approach.... You and I have chatted in previous videos that we might send out that new encounter building in an Unearthed Arcana... Right now we are focused on playtesting that internally instead..."
"What we have discovered is that just us iterating on it over and over and over again is bearing amazing fruit. What I can report is that what people are going to see in the revised DMG is a much streamlined encounter building system.... where you are able to figure out your budget for monsters.... and you spend that budget...the end...it's going to be that simple of a process. I'm looking forward to us sharing that with people later this year."
Basically, there's no plan that he's sharing to playtest monsters/encounter building externally. Maybe they'll do it, maybe they won't. Sounds a little like it won't be public until the DMG, but that's trying to read tea leaves. Up till now, though, yes it has been exclusively internal. That's the important bit - what's happening functionally.
The issue there is...
(a) With all the heavy design work done for the classes... it was being done in absence of external feedback on the other half of the game – the monsters/challenges/GM-facing stuff that the classes are built against. That's a huge problem, in my eyes.
(b) Secondarily, the context of this issue is that they (WotC) have a history of botching monsters/encounter building - so it's an area that needs attention, and many many D&D fans know it needs attention. We have Mike Shea, Teos Abadia, Shawn Merwin, Mike Mearls – all commenting about these issues, and publishing resources to address them.