They mostly likely have had contact with some people. In the first photo, you can make out a nice machete and a pot.
Having this tribe get a machete and pot and see a plane makes it feel pretty 'contacted'. Not that I want to quibble over some definition...
I think that they mean that these people have not been in direct contact with non-tribal people. It sounds like most of them are very isolated. Even the other tribes that have contact with the mainstream society seem to have little to no contact with these "uncontacted" tribes.
This is mostly info I read on that website and related links, i.e. I'm not an expert.