First of all, they signed him in 2017, after a period when they had been unable to sign contracts due to sponsorship trouble. When these problems had been resolved, there were not many quality riders on the market anymore, so they had to do whatever they could. They even signed riders like Julian Cardona or Kim Magnusson who very quickly turned out to be not WT level at all. But if you enter the transfer market in September, that's how it goes.
Secondly, I feel it's harsh to conclude he is not a WT sprinter. He finished 3rd on one of the stages of his first ever GT at TdF 2016, with Kristoff, Laporte, Sagan and Groenewegen on positions 4-7 that day. He won a HC classic after an almost
legendary sprint. He was perhaps the best rider of a decent PCT team. Even at EF he was not a complete failure IMO. He gave them 2 wins in 2 years, which puts him on par with any quick guy the team has had in many years. He almost took a WT win last year in Poland. Ultimately, he is not consistent enough to be a lead-sprinter of a good WT team but I would say that his early-career results definetely earned him a shot to at least try himself at WT level and I dare to say he would have taken a couple of WT wins by now with a semi-competent lead-out. He is a borderline WT-PCT rider IMO and looking at how Slipstream had lacked a competent sprinter pretty much since Farrar's decline as they were putting their priorities elsewhere, it made sense to give it a try.