It's a Myth that Ledgendary Raids won't get you a Ex Raid invite.
I know of 2 people that got an invite and they only did a Ledgendary Raid at the Gym the Mewtwo Raid was at.
Also to add, not everyone who Raided that Gym got an invite either. Talking to one of the 2 that got his invite yesterday said that 3 of his mates that were on the same Legendary Raid never got an Ex Raid invite.
A lot of lucky required to for these:
1. Lucky to do a Raid at the chosen Gym.
2. Lucky to get the invite out of possible 100's that have done a Raid a said chosen Gym.
3. Lucky to be available in the time/date the Raid is scheduled.
4. Then the Luck of capture once you've won the Raid.