The problem is that this is not accurate.

They are not in one place. Several (most? we don't even know) bands have sought safety in Seanchan lands, not all of them in Ebou Dar specifically. There is a large group in Ebou Dar, but it's not the only group.

Earlier on, we also have heard that a great many bands of Tinkers had sought safety in the Waste (Many Aiel touched by the bleakness were going to them). It's a distinct possibility that those Tinkers are still in the Waste, if they have not been convinced by the "bleak" Aiel who joined them to return to Rhuidean. With Aviendha going there and in the columns, the possibility that Tinkers bands will be nearby or close enough that she could find them is a distinct one. Not certain, but a possibility. These three books are the "last book" and Jordan had to bring things together. Rand has not seen the possibility of Seed Singing, even though he is aware of who the Tinkers are. Aviendha is about to make the same journey, so perhaps it will come from her. The best and fastest way to teach the Tinkers abou their past and what they call "the Song" is actually part of is to send the Madhi into the Glass Columns. The Aiel have there the means to show the Tuatha'an what they have been looking for, while only the Tuatha'an can teach the Aiel the Way of the Leaf.

That would have to be a decision made by Wise Ones. Aviendha is about to become one, is not as burdened by traditions and taboos as the older ones and she is right there in Rhuidean and she is acutely aware of the food problem, touched by the misery of the refugees in a way she herself doesn't fully understand. ("They are not Aiel, why is it I still care for them?" It's easy to see this as foreshadowing that she could be involved in finding a solution, or that before the end she might be the one who sees beyond strictly the Aiel to see the needs of humanity as a whole and what part the Aiel/Tinkers may play in it.

Secondly, that's also wrong that this is the first time the Tinkers gather like this. They have held similar gatherings of the bands each year around Sunday for millenia (the first time we heard of Tinkers gathering in the south, it was actually dismissed by a character as just that "oh, it's just their annual Sunday gatherings". The only difference now is that they don't gather between themselves for Sunday but have sought the vicinity of cities. Nothing points (to the Forsaken, not to us who see all the foreshadowing) to the fact anything of any importance has changed about the Tinkers or that anyone is on the verge of discovering anything important. There's no reason to worry about them all of a sudden. The Shadow had plently of opportunities to eliminate Tinkers in the last three millenia if they gave a damn about them or their philosophy. To the Shadow they're just light siders who won't lift a finger to fight for the Light, the shadow of a shadow of the da'shain who don't remember anything of what they were.

They are even DF among them. If Moridin passed the order for these DF to kill all the others, they would not have lifted a finger against the slaughter.

No reason to believe the Shadow has now sent Trollocs to infiltrate enemy lines and go slaugther bands of Tinkers right under the walls of Seanchan-held cities. It's a very far fetched and implausible notion.