It doesn't follow that viral load at infection matters because the immune system takes a while to respond. You have a built in assumption that immune response is exactly the same whether there is 1 (arbitrary unit) virus vs. 50 (arbitrary unit) virus. I'm not sure that is true. I could imagine the body starting a heavy immune response at 100 virus--which would mean the difference between 1 and 50 might have little affect on outcome.
Your paper is about viral load at diagnosis--not infection. Diagnosis happens after your immune system has been fighting a while.
That study was not supposed to refer to viral load at initial infection, you need to test viral load at exposure in a lab.
The 1 vs 50 difference is known to be based in part on the innate immune system not just the adaptive immune system. Most of the time you don’t need to create specific antibodies etc. If only a single virus enters the body it’s likely for you to fight off the infection without noticing though with the downside of failing to develop long term immunity.
The 50 vs 50,000 is a related though less proven idea. Extreme exposure can simply overwhelm the immune system and extreme cases directly result in septic shock, but the middle ground is harder to test.
Your paper is about viral load at diagnosis--not infection. Diagnosis happens after your immune system has been fighting a while.