Weather seal la, means keeping elements, including dust out of the lens as best as they can.

That's just info for your consumption.

Go "filter-less" on those lens la, gonna have problem.

It is as what you said - "dust mostly get into the lens due to the vacuum in the lens when you zoom in and out". Those models, the front element moves as you zoom in and out so in order to keep dust out people install filter on it.

After some reading, you are right. It's a rather odd definition considering I still see people having all sorts of foreign stuff inside their lens despite "weather seal". Well, I'm not familiar with construction of weather sealed lens anyway.