Perhaps the chicken was done Chinese-style, fresh from the wok, rather than pre-cooked. That would give 'tender' & flavour too.
Use chicken breast straight from the fridge. You want it really almost frozen. I have a 'half-way drawer' in my fridge that keeps a temperature of between 0 & 0.5°C, so it can be very slightly icy as it comes out.
Cut each breast in half, down its length, then slice across. I use a fork and razor-sharp boning knife to do this. Hold the fork against the nearside of the chicken breast, 2mm from the last cut & draw the knife towards you, using the fork to keep your cut steady, through in one cut. (You get two pieces per cut this way.) It's no good if you have to saw at it, you'll never get it thin enough. The boning knife has very little drag, so the chicken doesn't try to follow you across the board as you pull through.
I did this for dinner just last night - wish I'd thought to do some photos, because it's easier than it sounds;).
Marinate in light soy, rice wine vinegar, sesame oil, crushed garlic if you like, for 30 mins or so.
Get your wok as hot as possible, add oil & stir-fry the chicken. It will take only between 1 & 2 minutes to get it 80% cooked.
Drop it in your still boiling ramen soup.
The chicken will continue to cook on its way to the table.
Most people over-cook chicken.