That's like saying you're fresh to play a basketball match after having spent the whole day playing soccer.
During his grass streak, he only made the RG final 3 times. And in those 3 times, he twice won Halle & Wimbledon: 2006 & 2007. In 2006, he played 31 matches leading up to the grass season and in 2007, he played 34 matches leading up to the grass season
Nadal played even MORE matches in the year he won Queens Club and Wimbledon (2008: 35 matches, and won the French). So it's not like it's impossible to play a basketball game after having spent the whole day playing soccer. Federer isn't even the only person to have done it. So like I said, the grass season being so short is a benefit, not a detriment.
It's clearly not at the level of winning a CYGS in the present day. Anyway having the highest Slam count is a bigger achievement imo, since Federer set the bar so high.
Regardless, Laver's CYGS is considered more impressive than Nadal & Borg's 4 RGs in a row and Fed's 5 Wimbledons in a row (and Sampras's 4 Wimbledons in a row)...even though there was very little surface variety. This demonstrates that incredibly high form in a short period of time seemingly appears to be more impressive than chunks of incredibly high form over several years.