I know it can seem difficult to compare players from one era to another but I'd have to agree with your dad.
Lee Roy Jordan not only played LB on defense, he played Center on offense. He also made 31 tackles in ONE game against Oklahoma in the Orange Bowl, which was a shutout, (17-0). Lest not forget he played 14 years in the pros and was part of what is considered one of the greatest LB corps in NFL history where his nickname was "killer" and he was the defensive team captain.
Coach Bryant said if the runners stayed between the sidelines, Lee Roy would “get ’em.” Among other superlatives from Coach Bryant, that's a pretty good endorsement.
The biggest difference when comparing players today against players around Lee Roy Jordan's day would be size but he was a 220lb LB, which isn't that small even by today's standards. However, there's not that much difference in LB sizes today versus Lee Roy Jordan's day, he was undersized then as well. The biggest size difference is in O linemen and D linemen. I don't think there is a speed differential, or if there is it's marginal and could go both ways.