Soil is not necessarily the growth problem.
Sunlight is a major factor. While it is the land of the midnight sun it is also the land of midday darkness. While that is not strictly true, there are also long periods of very short days. The summer very long days do not have the strong sunlight.
And then it is cold. Cold inhibits growth.
The cold and lack of sun are what I first thought of and what I understood the guy to be referring too. But then why the huge trees in some spots in the interior? Anyway - it was something that obviously made instant sense to me, but had never even considered it!