Alright I'm back.
Now he presents a very good argument for why the Forerunners would likely present a serious threat in current 40k. I'll go over the elephant in the room in a sec, but first I'd like to point out the weird ass handwave for Chaos. I don't doubt the Forerunners would be smart enough to see the obvious bullshit there, but to say none of them would fall is a gross underestimation of the forces of the Empyrean. Also he sleeps on Nurgle far too much. Like yeah buddy I'm sure your genetic engineering is gonna be able to overcome plagues that have brought genetically perfect specimens like Primarchs to their knees. The Destroyer Plague and Nurgle's Rot are not things you can just cure with science alone, you need to have an intricate understanding of the Warp and more than a little psychic power to even have a chance of surviving a close encounter with them. If the Forerunners didn't take the necessary measures, I don't see how this scenario ends any better for them than the Flood (and at least you can burn the flood into nothing, with Nurgle's Rot and the Destroyer Plague even that will probably not be enough).
To address the biggest point of contention I have here, I believe the person in the video is vastly underselling the capabilities of the ancient 40k civilizations. His analysis of modern day 40k going up against the Forerunners seems largely accurate (again, aside from the Chaos problems I pointed out), however the Forerunners would be vastly outclassed by any million year+ civilization in the Milky Way.
By my estimates they'd be about on par with Dark Age humanity. Humans back then had legions of Men of Iron that probably numbered in the quadrillions at least. They had an empire that vastly dwarfed the modern Imperium of Man. They could create works like the Phalanx (which is a moon-sized space fortress) and the Speranza, an Ark Mechanicus that's a gigantic secret STC factory, with a hyper-advanced A.I. that can crap out black holes, use a time gun to make the future and past versions of a target destroy each-other by colliding, and literally Ctrl Alt Delete space itself.
The battles of the final golden days of humanity were fought with sun-snuffers and mechanivores which could not only eat space-time and everything therein, but information itself. That whole point about Master Chief's armor being like a hazmat suit? Guess what Terminator armor, the be-all end-all of traditional Imperial armor was based on. That's right, nuclear reactor maintenance suits from the Dark Age. And knowing the Dark Age, Termie armor is likely still scaled down from its original capabilities as civilian job wear.
To claim that the Forerunners are comparable to the Necrons and C'tan at the height of their power is laughable. C'tan Shards alone are like focused stars, and possess a mastery over the material universe afforded to no other species at their prime. The Old Ones, beings who engineered an entire infinite dimensional construct inside the Warp, the Webway, had to create or uplift entire vassal races to fight against the C'tan and the Necrons. And they still lost.
The ancient Eldar created horrifyingly powerful Warp-based weapons that would come to be seen as the Eldar gods to combat the C'tan and Necrons, and they still couldn't prevent their victory. The Necrons technology alone is beyond anything the Forerunners could hope to match: they can reset the entire timeline if things don't go their way, skilled Necron technicians can play with the fabric of the universe like children. The Celestial Orrey, a Necron construct that has a map of every star in the galaxy and can snuff any of them with a flip of a switch, and potentially lead to a gargantuan chain reaction that might destroy the entire galaxy, is regarded as a Necron piece of ART. Not a weapon, art. One of the greatest works of art ever accomplished by the species yes, but still, that should give an idea what level the civilizations in the War of Heaven were operating at.
Hell one of the best theories for why there seems to be no life in the universe aside from the Milky Way is that the War in Heaven ****** up reality so hard there's only one place left with any notable life. The other theory is the Tyranids, but then me thinks they were just scavenging what was left after that whole shebang.
Side note, and this is really petty, but I'm still gonna bring it up: I don't like the shade he throws at Necron tech for failing to turn on properly, while praising Forerunner tech for running just fine, even though he quotes it as having been 100,000 years. Like, the Necrons were asleep for 60 million years. That's a 600 fold discrepancy. The Forerunners are not impressive lol
