How is this mild Picard spoiler possible?

Question

In Picard a familiar character asserts to Soji that

The Romulans on the artifact are the only Romulans ever assimilated by the Borg.

How is that possible? In TNG, Data says

The Borg activity in J-25 is identical to "what happened" to the Romulan outposts along the Neutral Zone. It is strongly implied that the Borg have had a hand in assimilating some Romulans. This question seems to affirm.

I'd buy the idea that this is a well-kept Tal-Shiar secret, except

Hugh is the one who makes the claim about the Romulans, and I'd think he would know if more Romulans had previously been assimilated or not.

Answer 1

It's unclear what really happened in the Neutral Zone, and if the Borg actually assimilated anyone

It might be a plot hole, but there really is no sufficient information on what happened. This is what we know:

The outposts weren't simply destroyed, they where "scooped off the planet", which is something we haven't seen on screen yet:

DATA: Captain, there is nothing left of Outpost Delta Zero Five.
LAFORGE: Must have been one hell of an explosion.
DATA: Sensors indicate no evidence of conventional attack.
PICARD: Can you determine what happened?
WORF: The outpost was not just destroyed, it's as though some great force just scooped it off the face of the planet.

(TNG: The Neutral Zone)

It's noteworthy here that this seems to have happened to multiple outposts without any sign of a "conventional attack".

We also know that these outposts have not been unmanned:

RIKER: Captain, I recommend we transfer all power to phasers, and arm the photon torpedoes.
PICARD: Wait. If that is a Romulan ship, they will read our intent. It'll force them into taking a similar posture. We don't want to engage in battle.
RIKER: Captain, this is sufficient evidence. Outposts have been destroyed. Countless lives have been lost.

(TNG: The Neutral Zone)

This is what happened on the Federation outposts. But it seems that this is exactly what happened with the Romulan outposts:

PICARD: It might be viewed as an act of aggression.
THEI: If our intent were aggression, you would not be here now.
TEBOK: If we go to war, let us be sure it is for the right reason. We are here because our outposts, which border the Neutral Zone, were also attacked.
THEI: Destroyed in the exact same manner as your own.

(TNG: The Neutral Zone)

This seems to be something in the standard repartoire of the Borg, even we don't see it on-screen, as indicated in "Q-Who":

WORF: Captain, the sixth planet in the system is Class M.
DATA: There is a system of roads on this planet, which indicates a highly industrialised civilisation. But where there should be cities there are only great rips in the surface.
WORF: It is as though some great force just scooped all the machine elements off the face of the planet.
DATA: It is identical to what happened to the outposts along the Neutral Zone.

(TNG: Q Who)

We simply don't know if "scooping up entire cities" does or doesn't include assimilating the people in it or simply letting them die. We know that the Borg prefer assimilating whole civilizations over individuals or groups, so they might just have been interested in the technology itself.

This might be some early installment weirdness to show how the borg value technology over individuals, scooping up entire outposts with no regard for the live onboard, in which case they just might have let the individuals on the outposts die.

Answer 2

For completeness I thought I should add this:

It's a plot hole.

The Romulan Orum was assimilated prior to Seven's liberation from the collective, in Voyager Unity. Additionally, in Infinite Regress, one of the personalities that emerges from Seven was a Romulan, indicating that at least one other was assimilated.

It's worth noting that the page for the Shaenor explicitly says that they are the only known assimilated Romulans, so it's possible that all the others disappeared under ambiguous circumstances (as in Philip's upvote worthy answer) such that the crew of the Shaenor are the only confirmed assimilations.

Not sure how Hugh didn't know.