Nope, unfortunately not. There are tongue and groove planks underneath it, but it's definitely particleboard. That's the subfloor throughout the entire house, so since we were only replacing a small area we were told to keep it the same for matching purposes. I have read conflicting reports on whether particleboard is suitable or not.

@HU_291315178 Rather I guess I should say it's not the structural subfloor. Maybe it could be called the underlayment? Or filler? Or the upper layer of subfloor?

I am going to tag @WeShipFloors and see if they can respond and help you.

Particle board will swell and then crumble. You need to remove and replace it with different type of material. Imperfections will telegraph through your LVP. You will be wasting time and money if, you avoid to do so.

I’m not able to tag anyone in these comments but I did want to say thanks for the insight and update for future reference to say that, as much as it pains me, we pulled out all the brand new particleboard and now we’re deciding whether to put down OSB or plywood. Just wanted to add that per Ardex/Henry, their leveling products are not approved to be used over particleboard but are approved for use over OSB and plywood. And per WeShipFloors, it is necessary to lay a plastic vapor barrier over any leveling compound that is used prior to laying down Supercore.