I don't think the WHO have ever said that there was evidence that you're immune after catching it once, they have said there was no evidence that you weren't. Given that antibody immunity is by far the most common pattern encountered with viruses the working assumption is that you'll gain immunity - it's just not being taken for granted.
I'm curious because the last two videos you posted seem to be simply restating that there's no evidence you'll acquire immunity after a first bout. They don't represent a change of position but do seem to represent a change of emphasis. Has there been a development to warrant that change of emphasis? I'm not aware of one.
It's also worth mentioning that antibody immunity does not mean you can't catch an virus. It simply means you have the antibodies in place to fight it off efficiently when you do. Generally, the body is so efficient at fighting off known viruses that you won't even be aware it happened.
