I'm sorry, but I don't understand what motivated you to wash all the soil off the roots. I sure hope it wasn't anything I
When a plant is being repotted, it's important to be gentle and disturb the roots as little as possible.
The reason the soil felt wet was because you were watering the plant too frequently, or something was interfering with the drainage. Either the soil was too heavy, or something was blocking the drainage holes, or all excess water wasn't allowed to drain off before the plant was returned to its drainage saucer or cache pot.
There are reasons for removing all the soil from a plant's roots, such as severe infestations of root aphids or some other soil-dwelling insect that feeds on a plant's roots. Overly wet soil is not -- IMO, anyway -- a sufficient reason to wash the roots clean.
I think it will be difficult to get soil back in between all those fine roots, without leaving any air pockets.
I do wish you the very best of luck with your repotting project. As I said, Spathiphyllums are pretty forgiving, so maybe your plant can survive this and thrive for you.
"Isn't it enough to see that a garden is beautiful without having to believe that there are fairies at the bottom of it too?" - Douglas Adams