The dialogue suggests that Syla calls Goliath. -> "Right Oli?"
Technically she can do stuff like that. She can pinpoint Goliath's location in the Dream World and echo some false memories. Perhaps she even has done this in advance because she knows who is after her so Goliath is always ready to come into her defense.
The problem is how fast Goliath appeared. At one point it's just Ymir, rising through the ground and wrecking havoc in the city. At another point, Goliath just materializes, grabs Ymir and sends him flying. It's impossible for Golaith to be in a relatively close distance and MC to have missed him. It's also improbable that he was underground like Ymir, because he just materialized on top of the city.
So while the dialogue suggests Syla calls Goliath, due to the way the scene plays out, my instict says that Syla either projected Goliath from her Memories, or she shapeshifted into him. (We know she can shapeshift).
Could this be proof that Syla exhibits the Power of the M43? Could be, but I don't find it likely.
It doesn't make sense to me that the Memory trait can mimic other powers so well, when the Body trait, which is the true shapeshifting trait, can't.
I think there is something else facilitating that ability. Some ideas:
Fantasy: Basically the monster mimics the trait. It causes damage but it's not "real". For example it looks like fire, but it's not fire.
A bit of all: The monster has something like 5% of every trait, and it's unique mutation allows it to mimic every ability it reads through Memory.
Lastly, I think the theory that Syla is a monster has pretty much died thanks to the Vestige.
Unknown "Only you, kinsman, carry our humanity on."
You "Only me? What about all the other humans?"
Unknown "Your question has been answered."
MC carries the Vestige's humanity in him. This pretty much translates to Syla being an alt-human, and MC is half human (from Stephen) and half seahuman (from Syla). Don't be surprised if Syla hides gills or some shit with her hair.
Perhaps strength eaters and the m43 are monsters that somehow are related to the sea-humans (monsterfication), considering the affinity for water, but how that could be, I can't tell at this point.
This could explain why MC, Syla and the M43 have issues with lightning. Water and Lightning is a bad combination.