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We are immune to all these old viruses and have incorporated them into our DNA code so really they are part of us and cannot infect us again.

Having copies of ancient viruses in our genome does nothing to confer immunity from them should they be 'resurrected'- the defunct copies are merely retroviruses that inserted copies of themselves into our DNA, but the copies mutated later and so can no longer be 'read'. In addition, the copies also have mutations that even if they were 'read', they would produce a broken virus. But these researchers produced a hybrid version of a group of these old copies that can be 'read', and as it turns out is free of errors that would render the virus nonfunctional. In other words, its a fully functional and infectious virus.

And having old copies of a retrovirus in our genome has no bearing on whether a resurrected version of it could infect us- most retroviruses insert copies of themselves for reproduction into our DNA at random locations, rendering any previous copies irrelevant. Even those that do show some specificity of insertion site show specificity only on the order of what region of a chromosome they insert into- even with an old copy or two in a targeted region there's still a surfeit of room for new, functional copies.