With something as complex as soil mix I'll bet the peroxide was fully gone within 24 hours. It reacts with anything it can.
It'll last longer in a petri dish and I've had a 15 year old bottle still be good, but in soil it'll be like a mouse fart in the wind.
If the soil werent saturated I'd say spray with peroxide.
It may be overkill (and too late for your current trial) but when using containers with no drain holes I like to provide a way to add or remove water without disturbing the soil.
I've used little pots, PVC connectors I found, medicine bottles with holes drilled around the bottom, and rocks arranged to make 'canyons'. This way a baster can suck out excess water and a beaker can add water without disturbing the seeds.