It has the same gameplay style, the same themes, and a similar setting. I don't see why it shouldn't be part of the series. It's not like you can create a sequel to a game like Deus Ex 10 years after release and market it exclusively to Deus Ex fans who still play games, which would be like 5% of the total video games audience.
It's not like they're demanding that the original game be taken off shelves and replaced with their game. Sucks how people are criticising an upstart dev like EM for taking things in their own stride when the car crash that was Invisible War was invented wholly by the very same minds that made Deus Ex.