answer by Alexander@sopuli.xyz is correct, but I just wanted to add some stuff.

firstly, why they recommend dry wood is that with dry wood, hardener would be sucked in by capillary action. but that is assuming along the grain. perpendicular to it, wood, or plant cell are still filled with water, and if driedthey shrink. but they can not shrink much mostly because the outermost dead layers do not change size, and would help hold the shape. with shrinkage, there would be gaps where fluid could seep into.

also, if you do apply to wet wood, same cells will now try to swell up, but same dead cells will stop from expanding. this will restrict the max amount of hardener that can be be absorbed.

now for some chemistry bits, basically diffusion will occur. In fluids it is called osmotic process/flow, where solvents may move across semi permeable membranes to reduce concentration differentials. wood is not really semi permeable (it is, well, permeable), so both solvents (water) and solute can move. but solute molecules are still larger than solvent stuff, so they do not diffuse much fast (diffusion is roughly proportional to mass). if you have water inside as well as outside, some solute will move in by diffusion, but rate would be less as there is no mechanical pressure gradient. only chemical potential / entropy contributing to free energy. when you have less water inside, there is a mechanical pressure driving flow, and then solvent will drag solute with it. still it would form gradients (solute concentration would be lower as we go inside), but this gives a better shot for solute. And more time is better to ideaaly reach a equillibrium conc.