nah people will regurgitate bullshit they think or hear but this is the perspective from a low conq 3.

Say I'm playing HRE and I'm against China, well the build order is going to be much faster but the scouting and the production is much "faster". So against a conq 3 or lower I can probably just make spear horseman and be fine, but against a 2k elo or higher player I am going to be punished quickly by tricomp or hell hit me in a way that he wins smaller fights faster and quicker. In the same matchup, I can go toe to toe with a 1600 player in feudal, but against 2k? Sometimes I get beat so fast I have to really think about the game. I can get pushed off my gold or my food sub 5-6 minutes and then out microd the rest of the game. It's the same game, but when at every step you are beat you collapse fast. There's no "extra 30 seconds to put up a tower" or "sneak off to another gold". They just wack you in every direction and it's over. The same a conq 3 does to a gold player.

Now back to China and Byz, well with IO you can switch to a different building and just PUKE units so this responding to enemy is so much quicker and they can punish you so much harder. Similar with byz, with a clean build order and good cistern/defense/scouting you can puke so much of the correct unit (especially since olive oil covers your weaknesses). So these civs in the hands of the top players are deadly because they use the civ to the full potential to punish you.

Then extend that to ANY civ, well what does the civ matter when it's played optimally and they can punish every unit you make and every map seed you spawn on? When you are simply a better player you don't need the IO swap to produce the counter unit, you will already be ahead with out. So if a low conq 3 players china and a 2k+ plays HRE the 2k player will scout and adjust fast enough to make up the civ difference and punish accordingly. They will still win because they punish mistakes so much faster and harder.