A multiplex assay is a testing cartridge that goes into the RT-PCR machine and allows it to look for and compare multiple different dna sequences in a single sample
The original test was a single assay, programmed to look for only one sequence in a sample. There have been multiplex assays that distinguish between Influenza A and B for quite some time. They have since developed them to include Covid.
So this claim is quackery based on ignorance of the technology.
What part of they can’t tell the difference between Covid and the flu do you fail to understand?
The maker of the pcr test said it was not meant for Covid snd should not be used.