Does it mean that if you stick to a clean diet + weight training, cardio or HIIT exercises can totally be out of your weekly routine?

Specifically for your goal of "lean mass gaining and losing BF"

Yes. Not a problem. Many bodybuilders do the bare minimum cardio (5 minute warmups) because they don't like it. If you don't like cardio, don't do it. It isn't necessary to reach your goal.

Now, if your goal is to be able to run marathons or take part in a triathlon, then no. Those goals require high stamina and that would mean cardio.

However, just a bit of information - Unless you are an obese beginner in weight training, it is unlikely that you can GAIN mass while LOSING fat. Why? Because gaining mass requires a caloric SURPLUS, whereas losing mass (in this case, specifically fat) requires a caloric DEFICIT.

Once you are past the newbie stage (roughly estimated at 6 months by most people), it is practically impossible to build any mass while at a caloric deficit - you will maintain your mass which is great but you cannot expect any significant growth. That is why you will find serious bodybuilders all CYCLE their BULK and CUT phases and it takes years (not months) of consistent training to build up 10-20kg of pure muscle mass.

I"m insecure nao cos am wearing L too. Means you catching up to me in size. Sniff.