Mcenroe does say some stupid stuff, especially these days. However, you have to give the guy credit for some of his impressive predications. Mcenroe started the comparisons with Sampras right after Federer won his first slam. Looking back, it was pretty impressive.
Also, Mcenroe said earlier this year that the top 4 should not underestimate Isner. Within the next month, Isner beat Federer and Djokovic. I bet his ego swelled up after that.
Federer has been compared with Sampras since 1998.
Noone should underastimate Isner - that has been known for years already.