Hero Member = 500 merit airdrop
You are correct. I will just add that theymos after the launch of the merit system made some tweaks, which gave some hero members an additional 500 merits, making it 1000 in total.
I read somewhere that he did that in order to help those that were very close to Legendary rank but at the introduction of merit system they were short on activity. Reasonable decision as it's not easy at all to get 500 merit and I I'm sure those affected were in quiet a shock. I know I would be in the similar situation.