Ah yes.
Morgans calling Luffy "5th Emperor" is because he managed to deal the heaviest blow against Big Mom in decades. Kid got a few ships, Luffy managed to invade the Tea Party, which was said to be a known important event, ruin it, defeat 2 commanders, "subjugate" the firetank and sun pirates under his command, and his Grand Fleet sailing around the world singing his glories.
ALL of these factors combined is why Morgans decided to call him "5th emperor". The bounty is given by the WG, but the 5th emperor title came from Morgans.
That's what people have trouble understanding. The 1.5B bounty and the 5th emperor title are not related. They are each given by different people.
Which is also the reason why, despite having the same bounties, Luffy is an official Emperor, while Law and Kid aren't.