Cardoso has denied taking the drug and is awaiting the result of the B-sample analysis but the rider was pulled from Trek-Segafredo's Tour team earlier this week and has been suspended by the team pending the final analysis.
"I just can't really believe it. I'm hoping that the B-sample is going to be negative because I can’t imagine how you could be so stupid," de Kort told Cyclingnews at the Tour de France teams presentation in Dusseldorf on Thursday evening.
"It just makes no sense to me. We've spoken about it briefly [ed. at the team] but not a whole lot. We're dumbstruck."
De Kort added that he had mixed emotions on the subject – welcoming the fact that testing in professional sport works but reiterating his disappointment and frustration with Cardoso.
"It's good that some guys get caught as it means that testing works and deters people from doing stupid thing, but that it's a teammate makes it a bit painful."
"I was mostly surprised," he added.
"I was in a restaurant having a nice dinner and then got a message saying it was coming out any minute now. I can't repeat my exact words but I was pretty angry. What can you say? All we can say is that it's good some guys are caught. I really can't believe it's a teammate.
"You can't be that stupid to take EPO now. Everyone knows that it's not possible anymore because the tests are so good. It's really not possible, and doing any doping at all is ridiculous but doing EPO is just beyond me."
De Kort is one of the most outspoken riders in the peloton when it comes to matters surrounding doping. Now 34, he came into the sport at a far more volatile time, signing for the Liberty Seguros team in the mid-2000s. The squad, which later morphed in Astana, was littered with doping cases, and de Kort dropped down a tier after becoming disillusioned with the sport.
"It affected a lot of my career early on – my teammates going positive. I started with Liberty Seguros and everyone knows that it was terrible. I had horrible years and I remember the first year, that was terrible. I lost interest in cycling. I wasn't happy and I didn't like riding my bike anymore. I didn't really want to race anymore."
That's why I went to Skil-Shimano, a smaller team. I wanted to be able to trust my teammates and it really hurt me that I couldn't trust my teammates back in those days. I went to a smaller team and I found joy again and confidence in my teammates."
De Kort added that he had faith in the majority of the riders in the bunch, including Giro d'Italia winner Tom Dumoulin.
"I really believe that 99 per cent of the bunch is clean now. I know Tom Dumoulin really well and if he can win the Giro then cycling has cleaned up. Obviously there are still some guys hanging around that don’t know, can't change or whatever. Hopefully they'll get worked out of the sport, but if he's really taken EPO then I hope I'll never see him again."