I don't see how it is at all.

I see it as people reading way too deep into surface-level marketing. The marketing of the trailer with all the points with Thancred, Y'shtola, and the WoL/WoD not really being what people thought they were not as deceptive but as people overthinking it.

When people said that the whole "Master Matoya" thing was alternate realities or something like Matoya's in her body I knew people were over-thinking it. It obviously had no hints to it but people wanted there to be because reasons.

SPOILERS:

The whole "become what you must" is not deceptive, neither is the "become the warrior of darkness" since the WoL/WoD are just titles for Hydalyn's chosen. It's based off of the actions they do and not what their powers are. The "become what you must" line goes straight into several themes of the expansion, which is whether a given duty is a burden you must bear or a burden you must choose to bear. This is highlighted with Ryne and her struggle to separate herself from Minfilia and whether she wants to do what has been thrust upon her or if she just wants to be someone else.

As for the WoL's "become what you must" the story conflicts you as to whether the WoL should align themselves with the light or dark given how horribly the first has fallen, and all the horrendous things that happen involving the lightwardens, the sin eaters, and how the light has destroyed most of the world as well as the revelation Emet-Selch gives you into the origins of Zodiark and Hydalyn. In the end you were supposed to get that it doesn't matter which you are, you must take the stance of whatever stops these calamities to stop the rejoinings and the mass loss of life at the Ascian's hands as much as possible. In a way, the best way to summarize it is the song Shadowbringers. There's a lot about it that ties into the themes and narrative of the expansion, but two major lines specifically must be mentioned. The repeated "One brings shadow, one brings the light" and the lines that immediately come after them. It's the line that blurs the line of who they are talking about, they could be talking about both the WoL and Arburt in that both did that on The Source and The First. But then, the last time the line is said it's "I am shadow I am the light" confirming that they are one in the WoL.

They may have exaggerated, but any "deception" is on the player's part in the overhyping of small moments in the trailer that's used to be a prelude to the events in the game.