Bury the stink.
Alkalinity will be ok. Pipes are in cement all the time and it has lime.
Here is another idea where you do not use chemicals: bury it.
Dig out soil as far as you can reach. You can use a spoon or whatever you can get down in there. Or you could loosen up the dirt with a stick or retired screwdriver and then suck it out with a shopvac. Repeat.
Put sand in the hole, or clay or whatever you have handy or can find outside.
Stinkiest stuff in this hole is the stuff on top. That will be gone. Anything left with sewage in it will be under a layer of good sand / dirt / clay and it can stink all it wants under there.
If you cannot get down in there to dig you could fill the hole with water then bail it out into a bucket or siphon it out, over and over until the contaminated top stuff was debulked / washed away and you had room to put in the new clean sand or dirt.
No need for chemicals that might complicate things later. Burying has a long and excellent track record with stinky stuff.