As you say the horse chestnut leaf miner is a moth. The caterpillars make the mines in the leaves, then pupate in there. This lifecycle takes around a year to complete. If the leaves are isolated, say in compost bags, any moths that hatch from the pupae will be kept within the bags until they die, so taking the leaves out of the bags in August is safe. If your leaf mould has not been added to for over a year, it should already be free from moths.
However, this problem is so persistent now that you shouldn't be surprised if it returns. Most trees in the South East of England seem to be infested now. I understand that good controls are being developed but these may not be available to home gardeners even when they come onto the market.
In summary, cover your leaf mould until August - if you've been adding to it in the past 10 months. Otherwise it should already be free of moths at this time of year and fine to use in your garden.