I'll try to give you more insight. It's a bit murky what's going on to me as well, but here goes.
The reaper virus allowed for the Geth to evolve to a true organic-like intelligence. When Legion absorbs the code from the dead reaper he is upgraded, signified by his referring to himself as "I" instead of "We".
He is unable to TRANSMIT the reaper virus to the other Geth likely because of Legion's programming which makes him unique compared to the rest of the Geth (the 1183 Geth programs referred to in the comment below. Since the other Geth don't have all of this code they can't install the reaper virus until they are upgraded.
Legion is able to "go to them" and upgrade their software by contributing his own programs to the Geth network. So in one way Legion dies ... there's only a physical shell that no longer contains the essence of Legion. Once he uploads himself to the other Geth he's no longer Legion either, rather he becomes part of their programming. I think of it like wrapping a pill in bacon to give it to a dog. The "Legion" part was allowed in and it brought the reaper code in with it.
I hope this helps clear up some of what is going on in this scene.
Picture of the line in question