He chose the last 5 seasons because Underwood's first two seasons here were rebuilding from the Groce era in 2017. The 2019-2020 season marked the beginning of the Illini's return to relevancy. If that's not convincing enough of a reason, then we can choose Brad's entire career here since he took over in 2017:
Record: ILL 142-87 (.620), ISU 120-106 (.531)
NCAAT Appearances: ILL 4, ISU 4
Sweet Sixteens: ILL 1, ISU 1
Second Round Appearances: ILL 3, ISU 2
Conference Championships: ILL 3 (1 regular season, 2 tournament), ISU 2 (0 regular season, 2 tournament)
Losing Seasons: ILL 2, ISU 3
AP Poll Finishes: ILL 4, ISU 2
---> ILL: NR - NR - #21 - #2 - #19 - NR - #10
---> ISU: NR - #24 - NR - NR - NR - NR - #4
I think his point stands that there's no reason to think that ISU has a substantially richer history whether you look in the long term or short term. Going back 10-15 years would be cherry picking, and in response I would suggest to go back and compare the programs over the last 25 years.
All this to say, the original intent of his post was that regardless of certain ISU forum posters' beliefs, the Illini do have a comparatively rich basketball history, even if you only examine recent history.