Driver series has been on a hiatus ever since Watch Dogs appeared and it was previously known the former had been scrapped in favour of the latter but their nature now seems to be even more intertwined.
Driver series was using elements similar to GTA games ever since the first entry and built its success on them early on. However, when GTA entered the 3D market with GTA 3, Driver's popularity started plummeting and the devs struggled to give the series its own identity. These issues would persist through multiple parent companies and would only be fixed with Driver: San Francisco.
The next big game in the Driver series was in development but the game was scrapped and the engine would end up being used for the original Watch Dogs. That was the extent of their interconnectedness the public was privy to until recently when a fan of the series apparently got a hold of a Reflections team member to tell them about the history of the franchises.
Driver: Nexus was allegedly in development "alongside and just after" Driver: San Francisco where the series' recurring protagonist, Tanner, would be arrested for murder and later pulled out by FBI to help them with an undercover mission, where he would assume the name of Aiden Pearce. After the game was "50-75% complete", the devs released the gameplay debut which the higher-ups liked too much.
This caused them to change the direction of the game, scrap all Driver plot and references and make it completely based off of that trailer, which ultimately resulted in Aiden getting "a weak story and weird motive" that plagued Watch Dogs alongside the visual downgrades.
Now, the mail that allegedly came from a dev intimate with the grassroots of Driver: Nexus, later Watch Dogs, does not have a name connected to it and the game's weak driving systems made fans question the trustworthiness of the claims but considering the game had its entire concept and story changed mid-development, one could say this is plausible.
After all, it's been known for years that a Driver title was cannibalised by the first Watch Dogs, it just remains to be seen by how much.