According to the report, the superstar trio -- plus late-era Nirvana guitarist Pat Smear -- gathered at Grohl's suggestion recently to "jam" and the former Beatle said they should "just make something up."
McCartney, it appears, had little knowledge of Nirvana's history or influence. Cobain committed suicide in April 1994, with the band having released three studio albums and two live sets.
"I didn't really know who they were," he reportedly said. "They are saying how good it is to be back together. I said: 'Whoa? You guys haven't played together for all that time?'
"And somebody whispered to me: 'That's Nirvana. You're Kurt.' I couldn't believe it."
The Sandy Relief concert airs tonight on numerous TV networks and streaming services. Previously announced artists include the Rolling Stones, Eric Clapton, The Who, Bruce Springsteen, Kanye West, Eddie Vedder of Pearl Jam, Bon Jovi, Chris Martin of Coldplay and Alicia Keys.