This would seem impossible unless two different men named Sigurd Hart were involved – given that most sources suggest that his father was active in the late 9th century, which would make Sigurd Hart about two generations younger than Ragnhild Sigurdsdotter.
The author is suggesting a discrepancy between the age of Sigurd Hart and Ragnhild Sigurdsdotter based on available information.  They propose that there might be two different men named Sigurd Hart, one who was active in the late 9th century and another who is younger.