Hambini videos feature frames that have been sent to him because there was a problem discovered or suspected by the original purchaser. AFAIK, he doesn't buy randomly selected samples from bike shops to test. He is critical of the manufacturers represented in general, but it might be best to assume the criticism can at least be assumed to indicate QC departments that let a few 'bad ones' get through.
My experience from working for a company importing bike frames and parts from Asia is that there are two main issues at play - the manufacturing tolerances (which you can specify when ordering parts from manufacturers, but you pay more for tighter tolerances), and the ability of the final seller to catch and remove 'defective' items before they are sold. These two things are related in that you can order parts with looser manufacturing standards, but the percentage of defective parts will be higher. The reverse is also true - you can pay a premium for the Chinese factory to build your parts to tighter tolerances, and that puts the onus on them to catch most of the ones that don't meet the standard (and then they likely sell the 'defective' parts out the back door of the factory unbranded or as counterfeits).