Nothing about Concealed restricts its effects when a creature is Grabbed (unlike in P1e). You are "targeting" the grappling creature (from Escape:)
You attempt to escape from being grabbed, immobilized, or restrained. Choose one creature, object, spell effect, hazard, or other impediment imposing any of those conditions on you.
And the [Attack] trait is simply
An ability with this trait involves an attack. For each attack you make beyond the first on your turn, you take a multiple attack penalty.
So the letter of the rules is that this is accurate.
Pathfinder 2e is pretty explicit that GM's should lean on ruling in line with what makes sense. For instance, related to grappling, an example ruling is that you can target a creature anywhere their limb is while grappling, even if their "hit box" is not within reach.
Sometimes part of a creature extends beyond its space, such as if a giant octopus is grabbing you with its tentacles. In that case, the GM will usually allow attacking the extended portion, even if you can’t reach the main creature.
With this in mind, it is up to the table whether to apply this ruling or to allow a special case ignoring of Concealed's normal effect. It may even be case-by-case. A humanoid grappling you with one hand might be easier to ignore Concealed than an octopus who has multiple tentacles free to try to cling to you.