I think it is logical that Jakob´s best disciplines will be the long runs. His training has been almost only aerobic exercise for many years and that is also where his talent is (he is not a sprinter).
He has been training aerobically from a very young age being able to run 8k in 30 minutes when 10 years old. From around the age of 13 he has been training after the same schedule as his older brothers but initially only 50 % hereof.
His mileage has gradually been increased during the years but first recently to the same level as his brothers. But it is obvious when yo see him compete that his greatest asset is his aerobic capacity. So his training has really been more suitable for the 5000m than the 1500m as also pointed out by the brothers themselves.
When Jakob feels confident about his current shape and future possibilities I am quite sure this it is based on the times/ numbers from their training, especially their repetitions which are run at lactate number 3.2-3.3 (well under the treshold to anaerobic exercise). Many years ago when Henrik was still in his teens he ran 10-12 1000m repetitions in just under 3:10 per km. Some few years ago they ran the same 1000m reps in about 2:55. But last season Jakob and Filip ran even faster (I guess around 2:50 per km). Henrik could not follow the youngsters anymore.
I think that Jakob is running even faster at the same lactate number this year and that is the basis for his optimism.
The times of his 1000m reps have a close correlation the socalled treshold pace(treshold to anaerobic exercise) which is the pace you can sustain for 1 hour without "drowning" in lactate.
Jakob´s treshold pace has been measured by a Norwegian professor since he was 10-11 and his numbers have - considering his age - been extreme year after year. The numbers have been kept secrete for the last 3 years but it was rumoured in the beginning of 2018 (when he was 17) that his pace was between 21 and 22km per hour. My guess is that his treshold pace now is above 22km per hour. So it is not out of the blue that he claims he will do "extremely well" in the half marathon. 22km per hour equals a half marathon in under 58 minutes.