Worth of note that there isn't anything in the manga stating that, for someone to be considered an Emperor, they must match the power level of the already existing ones/their crews.
Morgans was never under the assumption that Luffy was as strong as Big Mom when he made his claim that he was a Fifth Emperor, there wasn't anything in his news to suggest that either. It is pretty clear this requirement is entirely on the fans, not something that exists on the OP universe.
Based on that chapter, we can tell that being a very strong pirate is a requirement, but not "being as strong as _____". A huge bounty and defeating strong enemies seemed to count as much as Luffy's numerous acheivements and the fact he got many pirate crews under his flag.
Also, Morgans using fake news to hype Luffy was something that only existed on the misleading spoilers. The spoilers suggested Morgans said Luffy had defeated Big Mom, but when the chapter came out we saw that didn't happen and every bit of information Morgans used was true.
One of the main reasons I decided to avoid spoilers from now on, mainly the spoiler discussions, is exactly because of this. A lot of people draw wrong conclusions based on misleading spoilers and those are crystalized within the fandom, becoming "facts" even if the real chapter denies the whole thing.