Except for all the uncertainty that still remains about the antibodies from infection and whether they protect people against which variants and for how long - remember Gaviria! - and relying on it would widen the unknown error range of estimates.
First - you raised my morale by providing Gaviria as an example: I could go and enjoy some stuff about cycling.
Second, ref 'uncertainty' - yes but minute. Here's an excerpt from the
Cambridge Independent online
“Of nearly 30 million cases to date since December 2019, there have been
only about 10 documented and confirmed
cases of re-infections. These data suggest that resistance to reinfection might be less a function of durability of the immune response and more one of breadth,” said the authors, led by Deepta Bhattacharya, an immunobiologist at the University of Arizona College of Medicine.
“Although the SARS-CoV-2 genome is diversifying slowly compared with other more mutable pathogens, high levels of pre-existing immunity in communities could lead to the selection of rare viral variants that evade neutralising antibodies.”
So I think it is entirely reasonable to make the assumption that for a fair period (could be less than 9 months at the 90th percentile) aka: "rely on it" that the risk of reinfection is nil, for broad statistical work. Can always foot note an optimism bias.
Off topic: Thank you for steering to the edge of the topic and sharing your "cycling through a zombie apocalypse" experience.
ETA: On the other hand PHE SIREN study says (press release):
"PHE scientists working on the study have concluded naturally acquired immunity as a result of past infections provide 83% protection against reinfection, compared to people who have not had the disease before. This appears to last at least for 5 months from first becoming sick.
"While the SIREN study will continue to assess whether protection may last for longer, this means people who contracted the disease in the first wave may now be vulnerable to catching it again.
"Between 18 June and 24 November, scientists detected 44 potential reinfections (2 ‘probable’ and 42 ‘possible’ reinfections) out of 6,614 [NHS] participants who had [previously] tested positive for antibodies. This represents
an 83% rate of protection from reinfection.