That does help, the rock isn't porous enough to have large dying worms inside it that looks like the type of rock we labeled unverified group b (not leaking ammonia or the worms wouldn't be opened as they are)
The worm accreted to the rock, with an extended crown showing its live, does take a few mos submerged to adhere to the side of the rock but these rocks may have been cured in full darkness, or in a vat with literally zero coralline seeding to begin the transfer to the rocks and make them obvious group b. Based on pics and details, if your tank was mine I'd treat it as group a and verify digestion ability to know when your cycle ends.
We go on and on about not killing hitchhikers with free ammonia spikes, but you have few and vs guessing with that much non coralline rock I'd simply digest test/cycle w dr tims as we listed. It probably already has some inherent bac, recycling it to verify as a stress test won't hurt.
Regarding the Api the choice is clear, proceed on with it and see if it's possible to use or get a salifert ammonia for thirty bucks and be sure of the reading. Api isn't bad and can indicate level changes, but getting it to read zero accurately, and know when your cycle is done, is the challenge.
A helpful aspect of the thread regardless of brand name used is that only what ammonia does matters, so there's only one param to bother with.