Is American Athletic Conference considered "Power 5." At best it's the 7th best conference after the football Power 5 and Big East. By that definition, you are correct - Arkansas '99 was Iowa's last win against a team from a conference that anybody cares about, and our record ain't so good against the other conferences, either.
Since Iowa's last Sweet 16 appeareance:
'01 - W Creighton (MVC), L Kentucky
'05 - L Cincinnati (CUSA)
'06 - L NW State (Southland)
'14 - L Tennessee (SEC)
'15 - W Davidson (A10), L Gonzaga (WCC)
'16 - W Temple (AAC), L Villanova (BigEast)
'19 - W Cincinnati (AAC), L Tennessee (SEC)
'21 - W Grand Canyon (WAC), L Oregon (P12)
'22 - L Richmond (A10)
'23 - L Auburn (SEC)
0-6 vs. "Power 6"
5-4 vs. other conferences
That was excruciating to type.