In Spider-Man No Way Home, we know that Doctor Strange's spell was botched mid-casting, and that caused everyone who knows that Peter Parker is Spider-Man, to start coming into this Universe.
So, how does Electro come to this Universe, as he never knew that Peter was Spider-Man? This is also confirmed in No Way Home when he says, 'You are just a kid... yada yada yada...and I thought you would be black', which means he did not know before!
There's a throwaway line in the film about him "absorbing data". Possibly that data included Spider-Man's secret identity.
"I was whoopin' Spider-Man's ass, he'll tell you! And then, he caused an overload. I was stuck in the grid, absorbing data. I was about to turn into pure energy, and then... oh shit. I was about to die."
That being said, assuming this Electro is the same Electro from The Amazing Spider-Man 2, he might have overheard Gwen say Peter's name during their battle.
This would also explain why he didn't know that "Peter" was a white dude if all he knew was his name.
