IIRC Umbric and his followers were chased out and exiled because they repeatedly ignored orders from Rommath to abandon their research into the Void (fair enough, it's a dangerous force and they were in a rough place where that could have rapidly spiraled out of control in the worst way, just like it did in Telogrus). Once you're given the boot like that the country that kicked you out more or less loses their claim on your allegiance if you then go off and do something else.
Considering Umbric's whole thing was researching the Void as an alternative solution to overcome their mana addiction, this places it fairly safely in the BC timeline pre-2.4, so the blood elves are still trying to pull themselves together. We don't have enough to know where in that time frame it occurs, but it's safe to say (since In The Shadow of the Sun is also canon) the blood elves' and Forsaken hold on the Ghostlands is pretty tenuous, meaning Umbric and co. could take exile/refuge there with reasonable expectations they won't get any aid from the capital nor bother the blood elves' affairs and in turn be left alone to their research, as it wouldn't be worth the manpower or resources to chase them further south to the Plaguelands.
This is one of those situations where Lor'themar calling them traitors because he tends to have a very self-serving, selective memory as shown in In the Shadow of the Sun, Mists of Pandaria, and Battle for Azeroth.