That quote that gives AKS puukko's thickness as 6 mm is strange as the only reference that fits gives it as 5 mm. But even that is very thick for the type of blades I found under "AKS puukko". I don't have the book where the reference apparently is from so can't check directly. Anything over 3 mm on old working puukko is rare. Various fantasy knives and quasi military knives are not included as they might have anything. Presently I think anything over 4 mm is rare.
Some data, showing that "Anything over 3 mm on old working puukko is rare" is a misconception.
Found during excavations
1350-1450: Blade 89 mm, Length 19,5 mm, Thickness 3,8 mm, beveled edge 12 deg.
1400-1450: Blade 92 mm, Length 21,4 mm, Thickness 6,0 mm, beveled edge 16 deg.
1400-1450: Blade 81 mm, Length 21 mm, Thickness 6,9 mm, beveled edge 21 deg.
1450-1520: Blade 120 mm, Length 21,6 mm, Thickness 5,1 mm, beveled edge15 deg.
From museums - Early Puukko manufacturers
1888: Blade 78 mm, Length 15,4 mm, Thickness 4,9 mm, rhombic
Before 1885: Blade 68 mm, Length 15,5 mm, Thickness 4,6 mm, rhombic
Before 1900: Blade 69 mm, Length 17 mm, Thickness 4,9 mm, rhombic
1815: Blade 85 mm, Length 17,1 mm, Thickness 4,9 mm, rhombic
Before1889: Blade 96 mm, Length 17,1 mm, Thickness 5,1 mm
Before 1898: Blade 80 mm, Length 18,1 mm, Thickness 5,8 mm
Before 1889: Blade 97 mm, Length 18,6 mm, Thickness 5,3 mm, rhombic
Before 1885: Blade 77 mm, Length 17,5 mm, Thickness 4,2 mm, rhombisch
1838: Blade 117 mm, Length 26,3 mm, Thickness 6,1 mm
Before 1889: Blade 65 mm, Length 17,7 mm, Thickness 5,2 mm, rhombic
1850-1900: Blade 81 mm, Length 17,3 mm, Thickness 5,3 mm, rhombic
1841: Blade 96 mm, Length 20,5 mm, Thickness 5 mm
1870: Blade 87 mm, Length 22 mm, Thickness 6,8 mm, rhombic
1897: Blade 90 mm, Length 17,3 mm, Thickness 4,7 mm, rhombic
1868-1876: Blade 97 mm, Length 19,9 mm, Thickness 5,7 mm
Academic Karelian Society
1935-1942: Blade 96 mm, Length 20 mm, Thickness 5 mm, rhombic
Puukko from Hackman Oy
1970-1972: Blade 178 mm, Length 31,9 mm, Thickness 6,2 mm
Tommi-Puukko from Jukka Värinen
1999: Blade 94 mm, Length 20 mm, Thickness 4,7 mm, rhombic
Jagd-Puukko from JT Pälikkö
2006: Blade 107 mm, Length 29,2 mm, Thickness 5,1 mm, rhombic