There is absolutely no way one can claim "strong immune response in 2020 for the original 2003 SARS virus" unless it was actually verified by reinfection. I am aware of the article claiming that there is some kind cellular reaction to SARS, but first of all, it might be not enough at all to protect from reinfection, and secondly, it might be case of well documented cross-immunity from some common cold viruses.

Then why the reinfections? People still too weak after the initial infection? Different strains? I just want to understand but it's probably still puzzling doctors and virologists.

AFAIK there have been very few reinfections. Humans have very diverse genes and over 47 million people have been infected. It's entirely normal that there will be outliers whose immune system doesn't produce a response that lasts as long. The rest is the media engaging in fearmongering.