If it's treated plywood, then you really don't need any kind of sealant because treated wood is quite resistant to rot -- However, it might be a good idea just to keep the wood from absorbing water vapor and
weight.
Had you thought of it in time, it would have been a good idea to seal the UPPER part of the wood to prevent the gasses from the treatment from being in the inside air -- Be sure to air it out often before you use it, esp in the first couple of years.
BTW, if I were rebuilding an egg floor, I would likely use treated wood for the durability, altho it is heavier than untreated wood -- Likely, I would let it thoroughly season and then seal it on top (or maybe both sides) before installing.