I don't think he lacks intelligence. There are so many types of intelligence out there, generally you are good in some things while bad at others, you can compensate by studying but usually somethings are just easier for you while others are not. Ability to learn and speak languages is just one form of intelligence, just like math, social skills, creativity, musical skills etc. Being able to understand hockey (or any sport) is one form of intelligence as well I would say.
Jesse is going to get better at English over time, so it's extremely premature to judge what his "potential" in English is. He just didn't put the effort back in school because he was busy with sports and probably lacked the motivation like a lot of teenagers, and now he has to catch up to what he missed in school. Learning gets tougher as you age and while Jesse is still a young guy, it's going to take some time especially 'cos the groundwork hasn't been done properly. Judging by his interviews, he understands English well, it's generating speech that is causing him trouble. This is pretty common with Finns IMO as the school system teaches grammar, vocabulary and reading and listening comprehension but actually speaking the language is left to minimum. How are you expected to speak the language if you don't practise it?
Shortly, I think it's dumb to say someone lacks intelligence because they don't speak English well enough at the age of 19. People making that claim don't sound too bright to me to be honest.