It is not. It goes directly to your assertion:
the quality within one that isn't subject to arising or passing away.
Let me ask this in a different way: Judging from what you have said, nibbana exists some how as a quality within
, but if there are no arahants, is there still nibbana somehow existing in some way?
So you accept the validity of "there is that dimension where there is... neither passing away nor arising" but you can't accept "the quality within that isn't subject to arising and passing away" ?
Furthermore, that was a quotation of Ajaan Lee Dhammadharo, not my assertion.
And finally, the question "if there are no arahants, is there still nibbana somehow existing in some way" is yet another philosophical abstraction. It looks like you think how someone answers this question one way or the other proves something, or that there is a definite answer to this question given by the teachings, and therefore other things follow from that. If you can point to where this question is asked and/or answered in the suttas, then that would be worth knowing.
I agreed to the ending of dukkha is nibbana. The question is, what is the nature of what is being described in the Udana 80 text in question.
So now you don't agree that Udana 80 is talking about nibbana? Even though it says "this, just this, is the end of stress [dukkha]"? And furthermore, in accordance with your own reasoning elsewhere as to the context of the Udana passages, they are talking about nibbana, as they all start with
Now at that time the Blessed One was instructing urging, rousing, and encouraging the monks with Dhamma-talk concerned with Unbinding [nibbana].
"When one thing is practiced & pursued, ignorance is abandoned, clear knowing arises, the conceit 'I am' is abandoned, latent tendencies are uprooted, fetters are abandoned. Which one thing? Mindfulness immersed in the body." -AN 1.230