The studies suggest that not only do people have long term defences, but that many people's immune systems already had defences in place capable of recognising this virus. They're even able to distinguish between people whose immune system had pre-existing defences and those that only developed them after being exposed to SARS-CoV-2.Unfortunately your opinion and scientific fact suggests otherwise. The studies suggest the immune response is not consistent in terms of an innate or adaptive immune response in the event of re-exposure.
Inconsistencies are to be expected, and even for vaccines. So that does not argue against long term immune system memory.