Well Juncker said yesterday that UK can rejoin in a two centuries - maybe. EU leadership is against letting them back in, so I don't know where get that 'in a heartbeat'.
Juncker is a retired politician; he can more or less say whatever he wants. At the end of the day, the UK rejoining would be of benefit to everybody, and it's hard to imagine that much opposition.
2 centuries? Yeah no. 20 years? That's more likely.