You’ve asked this before. LHR is full, someone has to trade a slot for you to get one.
It’s called churn and had been a common feature in the 37 years since LHR was opened up to new airlines once again in 1991 after having been declared full in 1977.
Heathrow didn’t find new slots for this, the airline would have gone to market and paid a sum of money, or gotten some of the rare slots that go to new entrants.