I don't think anybody's denying it exists. What hasn't been shown is if long-term consequences are more prevalant with covid than with other viral lung infections.
As with any other early stage research relying heavily on self reported symptoms, the prevalence figures are not well established and the mechanisms causing it not yet understood, but they don't look like flu.
[1]https://www.bmj.com/content/371/bmj.m3981, https://evidence.nihr.ac.uk/wp-content/uploads/2020/10/Livin...
Long COVID is unproven and anecdotal and seems to be more likely to be psychosomatic in most but not all cases.
Or if it's even caused by Covid and not some underlying pre-existing pathology that ANY disease could trigger.