Yeah, there's a limited window over which antibody testing is useful. I hadn't heard that it was only a few months, but that wouldn't be shocking, especially if you want a low rate of false negatives. Hopefully someone reading this knows more than me about approaches to testing that might work over longer time scales and not be confused by vaccination.
Actually I just Googled again, and they simply say the precise duration is unknown [1]:
> IgG antibodies, including IgG against the S and N proteins, persist for at least several months in most persons, but the precise duration of time that antibodies persist after infection is unknown.
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