What cut of beef did you use, and what grade? The only reason I ask is because American beef tends to be of a much lower quality than Japanese you can tell from the video that, that slice of beef in the states would be considered USDA Prime based on the marbling which would have a significant impact on the tenderness. For a lower quality beef I'd suggest letting it sit overnight with the onions in the refrigerator.

I tried this once with a Prime grade tri-tip steak and it was delicious and suuuuper tender. Although I did take some liberties and grilled it to get a char-grilled taste so I missed out on the meat juices but I added a mixture of Soy Sauce, Mirin, and Teriyaki sauce when sauteing the onions and it ended up being amazing.

Also this is just my personal preference but try to cook the onions so their a translucent golden brown. It could just be the angle of the shot or the lighting but it looks like you might have added to much soy, or even overcooked the onions.

Also a secret tip for if you ever try this dish again, instead of using just soy sauce, prepare a special mixture ahead of time using Soy Sauce, Mirin, Sake, Apple Juice and Minced Garlic, and a Tiny bit of rice wine vinegar. Once the mixture comes to a boil throw in a little bit of Bonito flakes and let simmer for a minute or so and remove from heat. After you strain it use it as a sub for the soy sauce in the onion sauce add a little bit of cornstarch (like a very small amount) to thicken it up so it sticks to the meat and viola!

Also the remaind of the sauce can be used for Yakiniku (Grilled Meat) trust me it's great stuff.

And sorry if I come off as pretentious I don't mean to.