'In the eyes of people'.
He is the longest reigning Hokage, most of the konoha people saw him only compared Hashirama and Tobirama. During early naruto Hashirama level was completely unknown.
Also it wasn't exactly a 'official' title. Hiruzen title is like a rumour.
Whereas Mihawk as the strongest is not just spoken but in actuality + Narrator + Oda keep reinstating that.