We have a citation on the page, linking to a tweet from a member of CRWBY. In that Twitter thread, they were talking about Grimm that had appeared in the episode "As Above, So Below", since they could finally come out and tell us things about those Grimm since the episode had been released. Miles reveals the name of the Teryx, they talk about CRWBY nicknames for these Grimm, and Larissa mentions they had nicknamed the Megoliath creatures of Grimm "Manny". Since they were specifically talking about Grimm that had appeared in that episode in particular, this is confirmation that those Grimm are Megoliaths.
The Megoliath you're referring to looks the same as the other elephant-like Grimm, except bigger and more evolved, indicating that it's an elder Megoliath.
Are you sure the tweet wasn't merely a sort of hint of what's to come at that point? Because Megoliath being a new species instead of an elder Goliath like an Alpha or a Ursa Major just seems to be rather... over-thought? Strange?
Hopefully some Word Of God will show up soon to help clarify this, like Rwby Rewind or some concept art.
In the meantime, you must forgive me if I remain skeptical.
It is really strange that they gave these mammoth-like Grimm such a similar name to the Goliaths, but they look so drastically different from Goliaths that we felt it safer to make a separate page for them. Putting the Megoliaths on the Goliath page, despite the visual difference and the fact that the Megoliaths appear to be smaller than Goliaths, would be like saying "They definitely are Goliaths", which could end up being misinformation. Even the Solitas Beowolves don't look that different.
It's sort of similar to how we have separate pages for the Manticore and the Sphinx due to them having entirely different names and a lot of physical differences, despite some fans' theory that the Sphinx is an elder Manticore. Putting the Sphinx on the Manticore page would be saying "It's definitely a Manticore", which is worse than giving it its own page. Similar principle with the Megoliath.
We don't have enough to solidly say that they're the same Grimm, due to the differences we've been presented with, so it's safer to have separate pages for now.
I suppose it is safer, though the logic that the 'Megoliath' look different enough to Goliaths to be considered different is faulty, since we haven't seen Goliaths since Vol.3 and as we know Grimm species like Beowolves and Ursai got design ovehauls in Vol.4.
So... yeah. Here's hoping we get some clarification.