For 2018 Renault was the smart choice, insulating Red Bull from the risk that Honda would still be a distinct 4th best, meanwhile they can work with Honda and Toro Rosso towards having the Honda engine well integrated into the 2019 Red Bull. At which point based on rates of progress Honda may well have overtaken Renault.
That is the luxury of having a 2nd team like STR, Red Bull can really get the best of both worlds in terms of having the right engine for the right moment.