Rumor is that it was in development since shortly before Res 2 Remake released. Supposedly Capcom themselves aren't making this one, they contracted it out to a 3rd party studio. It will make use of Res 2 Remakes engine and reuse some Res 2 Remake assets to help speed up development as well.
According to one of the Resetera mods with insider info, he clarified today that it is being developed in-house for two years and a half already, just not by the same RE2 team. The same engine as RE7 and RE2 remake as expected, too.