What's your reference for this? All references I can find say that for travel within Nordic countries you don't have to carry any proof of citizenship with you, an ID like a DL is enough (and that's how it's always worked in practice). Within the Schengen area it's different, you do need proof of citizenship.
Nordic citizens
do not require passports or national ID-cards showing nationality for travel within the Nordic countries. It has been so since 1954 and the Schengen rules did not change that. SAS permits its passengers to travel within the Nordic countries by showing any kind of ID, such as a drivers license (without nationality) or a Swedish bank ID card (also without nationality). If any official doubts that the person in question actually is a Nordic citizen, the official will check the nationality with the respecitve country. I travelled ARN-TKU with a friend, who is adopted from an Asian country. I had my national ID card and my friend just had a Swedish Bank ID, without nationality. The Finnish border police did a random check that day and asked my friend whether she actually was a Swedish citizen. When they could hear that my friend spoke fluent Swedish they said "OK" and that was that.