their to big to be fungus gnat larvae,
when you mixed the soil has it been sealed after so nothing can get in to lay eggs, having a mix like that around would be humid and attract all types of bugs, im sure sub did his in big bins and had the lids on tight so nothing could get in,
so im not 100% sure what method you use to mix it up but i remember someone doing it with a big bag and ill have to go back and check if that was what you did, if it was sealed up after then i guess it can only be something that was in a product you used and hopefully its not harmfull, dont you have any veg lying around you could just put on top of the soil and see if they start eating it, slice a potato up and put it on the soil like you do when you have fungus gnats, if their going to attack the plants and roots then they will be all over a sliced up potato, at least then you will know if their living off the soil or the plant matter,
i would find out before planting just in case but a simple potato test should give you a good idea what they are likely to do.
you dont want to be adding chemicals now you spent all that time mixing and waiting for it to be ready, you could do with a big press and put the soil in then squash it down to kill everything in the soil, it will only kill larvae and bugs the microbes will still be ok, but its finding something to squash the soil in as it would need pressing pretty hard to kill everything, then would need breaking up and mixing up again,
you know what i think they look like, i think the look like worms you see near ponds and stuff like that, could it be mosquite larvae, im sure ive seen something like that before,