Unlimited Void mainly targets the mind, consciousness, and soul rather than physical speed. Based on everything we've seen so far, Sukuna is arguably one of the top one or two characters in terms of mental and spiritual resilience, yet he was still affected the moment Unlimited Void was activated.
With that in mind, I don't think it's unreasonable to say that its effect is consistent, or that Dabura would be affected by it as well. Tying concepts like consciousness, the mind, or the soul to movement speed doesn't really make sense.
So even if Dabura were Sub-Relativistic+, he would still be affected by Unlimited Void. I don't think that debunk is particularly convincing.