Yep the bits are great, just again remember you have to treat for a few weeks because it doesn't affect eggs or adults. Be cautious also not to let the bits sit and rot.
I bought a plug of utric recently that apparently had some fungus gnat larva, after all I've done to eradicate these buggers! So what I did with all of my sundews is set up a deep container of distilled and mosquito bits and soak the container in there for a few minutes so the whole medium gets infused. I will do that once a week or so (not for any scientific reason, it's just my best guess) and then put them back in their cache pots that have 1/4" or so of water.
It probably IS good to water from the top, as the larvae like to be near the surface, but you'd have to infuse and strain. I do that sometimes using a fine mesh drain strainer I got just for when I work with my plants in the sink (like this https://amzn.to/39QPQLB
). I pour from a wide-spouted watering can or you can use a bowl, whatever, into a regular one.