In the final cutscene they talk about how if they can’t be purged then Halo has to contain them. What if it’s more than just being immune to the Halo Array and permanently killing them isn’t possible? Surely the Forerunners could have just had some Guardians destroy the planet otherwise if it was just the Halo Array. He seems to be saying they just couldn’t kill them.
It’s not a perfect analogy, but think a bit like Guardians in Destiny how as well as their powers they are functionally immortal so can’t easily be dealt with. You kill one and eventually it re-materialises in a new body because Space Magic.
I was also reminded of a story from the Horus Heresy series when they talk of “learning the extent of what their kind can endure”. A character, Vulcan, was immortal and so his brother tortured him by repeatedly killing him over and over as he reincarnated a new body. Perhaps this is what the Forerunners did? They took the Xalanyn and killed them repeatedly to learn what the limit of their reincarnation was.
This might also mean the Harbinger isn’t dead and would come back at some point. It could also be why they’re so dangerous.
Which could be why the Harbinger believes they and not humanity should hold the Mantle. If you’re immortal you would have a chip on your shoulder and assume you are a superior kind of life form.
So it’s less them being immune to Halo specifically and more that this ability makes them immortal. Halo probably can kill them but they just come back anyway if you do.
Just thought, if the Flood caught these guys they could really abuse this. 