I think Dan's move is a calculated, informed step forward, I wish him the best in 2019.
Works teams are always refining, reworking & updating their engine components etc, what if Renault during testing have found more grunt, but have decided to keep the new engine specs under wraps till 2019, sure it goes against the normal thinking of a competitive F1 team, but Renault probably realise that this season is to far gone to warrant massive changes and have decided to concentrate on their 2019 assault as a priority, hence going after Danny Ric.
Signing Dan may well of been based on what they have in place for next season, and Dan may have been privy to that info during talks, after all when you want to catch a big fish you need to dangle the right bait. Sure Renault may not have any ace up their sleeve for 2019, but signing the best overtaker in the field with the 2019 front wings producing less dirty air there's a good chance Dans skill will make up the short fall on power. But I think there's more to this than any of us can begin to speculate on.