IF they would be invisible, then yes. But it doesn't seem like they are.
We are back in "spells do only what they say they do" territory, and the spell says:
You flourish the weapon used in the casting and then vanish to strike
like the wind. Choose up to five creatures you can see within range.
Make a melee spell attack against each target. On a hit, a target
takes 6d10 force damage.
And even if it does say that you vanish, the first sentence seems to merely be fluff. It doesn't say that you attack in any manner that would give you advantage on the attack.
As stated by the top rated answer in the linked question, and the Jeremy tweet linked in there, it does not grant you invisibility.
Like everything, the DM could rule that you DO turn invisible, in which case Truesight would negate the advantage, at least RAW.
But "vanishing" could also be disappearing physically. Now you aren't invisible, in fact you aren't there at all.
This – at least to me – seems to be the intended function of the spell.
It makes you do a series of fast teleportations between enemies, and attack along the way. So to actually attack, you need to reappear.
The fact that we are doing a series of teleportations is reinforced by the last row of the spell:
You can then teleport to an unoccupied space you can see within 5 feet
of one of the targets you hit or missed.
"I'm sort of asking can Truesight see stuff that's unseen but not invisible?"
In what ways can you be unseen, but not invisible?
As far as I know, only in two ways:
- Physically obstructed, "hiding" – behind a wall, in a bush, so on.
- By not being in range of vision.
Truesight helps with neither of these, unless the target is hidden by magical darkness, or outside your Darkvision range.