"D. None of the above" - most likely.
Remember the situation. They (the mfg, and FDA/CDC, etc.) wanted/needed to get these "at home tests" out as fast as possible, for a variety of reasons.
But they had to do the regular testing, etc., to make sure they were safe and effective, etc.
IF they wanted to be *sure* they'd be effective, say, 5 years later (or 3 or whatever), they'd need to test them 5 (or 3) years later. That would hold up the entire distribution, at a really difficult time, when easy and widespread testing was a concern for a variety of reasons.
Yes, there are other ways to "speed up" testing of whether the tests would still be effective "later", but those methods are usually just proxies, and not always totally accurate. (Could be things like keeping the tests for a shorter time, but subjecting them to greater temperature fluctuations, or light, etc.)
I do NOT have any first hand experience with this type of test kit, so my thoughts here are simply "educated guesses", plus my thoughts are free, so you get what you pay for, etc.
Anyway, I assume that as testing continued, they are learning that the tests continue to be useful even longer than what they had initially determined... for timing that was relatively urgent, so they preferred to get the tests out there with an expiration date they "thought" was shorter than if they had been able to test longer...
Giving "them" the benefit of the doubt here, they weren't "kidding" and they weren't "make stuff up".
They were being conservative with efficacy and safety in mind... until they had additional results.