Why does one have a mild or serious illness? Is it the body's defense that determines that. Until you answer that it doesn't matter.
If an infection was asymptomatic or mild, it could be that the initial dose was very small, so the body's defense wasn't needed to kick into high gear to fight off the virus. That could mean less protection in the future. It could also mean a robust immune response fought off the virus. Part of that could have been previous infection with a somewhat related coronavirus helped prime their immune system so better protection. We don't know.
A serious infection could mean the person had a high viral infection early so over time they needed a robust immune response. It could also mean they have a weaker immune system that led to longer illness.
Finally, you need to have age matched and health matched comparisons. Those who are older and with more heath issues were the first groups to get vaccinated. Data indicates protection wanes with time, so time of vaccination and infection need to be factored.
There is no simple definitive answer yet. My point is that some may look at a report that previous infections may be more protective than a vaccine and that has a lot of bad outcomes. Some may take the view that it is better to get infected than to get vaccinated so decide not to get vaccinated. That in my view is absurd and dangerous. In addition, that report also says getting vaccinated after being infected provides better protection.