Here's a bit of an answer – Of course we're without research here, even the newest research doesn't really cover this particular question.
In my experience, having a withdrawer that's reassuring is different than a withdrawer that is reaching for/revealing their own needs.
Reassuring and reaching (revealing) are two different components of WRE. It's often easier for withdrawers to reassure – I think it's similar to a placating move.
Many pursuers will soften with a partially engaged withdrawer, ie. one that can reassure. BUT, a withdrawer (dismissive avoidant) that can stay in the face of fire and assert – that's reassurance of a different type – that's reaching or what I call revealing. Pursuers can know they are really important when their withdrawer reveals/reaches from their own internal experience. In this assertion they are doing “opposite action” in the face of the trigger. Staying and revealing. Now the P can really know where they are.
One caveat – if you've had a very defensive withdrawer (avoidant/anxious) who often counter attacks as a way of defending themselves (rather than just shutting down) the reverse pattern is helpful – to be able to stay and reassure rather than reveal/reach because the reaching/revealing can be so similar to the defensive counter-attack.
Finally many pursuers will not soften with a partially engaged withdrawer – being reassured is not enough for them – they want also to be needed by their withdrawer and help in containing their own trigger. You'll hear these P say – I want to be needed by you. I don't want to be the only one who brings up things and needs comfort. These P will start to feel shame, while their W can reassure they start to feel “why is it always me” that needs comfort. While they like that their partner can be reassuring they also recognize their partner is not yet doing a reciprocal move – asking for safety or comfort and asserting (reaching/revealing). In secure attachment it's reciprocal – both partners risk in these different ways: reveal, reach, respond and receive.