'um...' What? It's a fact the Adeptus Mechanicus mixes with new geneseed samples all the time. Take the 21st founding, also called the 'Cursed Founding' because of all the things that have gone wrong with the gene-seeds made at this time, such as the Black Dragons bony growths, and the Fire Hawks (now the Legion of the Damned) that had/have a nasty habit to spontaneously combust.
You've got some errors in your fluff and math.
First, There have been 25 Foundings (the First doesn't really count). 1/25 Foundings with gene seed that was "experimented on" is hardly "all the time", nor is it "most". "Most" Chapters are founded using a single gene seed type. To the point that it has been stated, for going on twenty years almost, that nearly 2/3rd are Ultramarines descended alone. 60-65% is "most". It would be accurate to say "Most Chapters are Ultramarines." We'll use 63%.
That leaves roughly 36.999% to be split up among the other original legions, the Spwolves being that .001% since they don't have any successful successors. So that's 37% split between 7 Legions, it leaves approximately 6% of Chapters descended from each one. Now, we know that's not the case. The Salamanders are significantly lower. The Blood Angels and Dark Angels are lower too. Blangels because of the Black Rage, and Dangels because apparently nobody trusts them. Any "Unknown" or theoretically "mixed" Chapters comes out of this same pool.
Either way, 1/25 Foundings was "Dark" and they don't know, that's a max of 4% but highly unlikely that it is all. 1/25 Foundings "Cursed" where some
experiments were made, another 4%. However, we know that of the seven named 21st Founding Chapters, two were single geneseed, and zero were hinted as being mixed, so again, significantly less than 4%. But then we look at the numbers. If even 8% of Chapters were mixed, that means there is only 29% left to split between the 7 other SM Legions. Averaging out to just over 4%. Kinda hard to believe that there are more "mixed" gene seed Chapters possible than non-mixed gene seed chapters. Especially since nearly two thirds are verified as single-source, and Chapters like the White Scars have no known flaws similar to the Ultramarines.
The reality is, there are less examples than a one armed man has fingers of Chapters who have ever been said to have "mixed" gene seed. A fully limbed person will still have digits left over after counting the number of chapters GW has even hinted
being of mixed gene seed. There's enough wiggle room in the fluff that the idea of mixed gene seed experiment chapters isn't impossible.
But "most third and other founding chapters have mixed geneseed" is flat out wrong, which is why he got the O_o face. And the idea that the AdMech mixes gene seed "all the time" is fairly silly, especially if you want to use that argument to refute the O_o face, lol.
Second, the Flame Falcons burst into flames. The Fire Hawks just disappeared into the Warp. It's a small point though.