Nibbana is an element (dhatu).
There are these two elements:
Dve imā, ānanda, dhātuyo—
the conditioned element and the unconditioned element.
When a mendicant knows and sees these two elements,
Imā kho, ānanda, dve dhātuyo yato jānāti passati—
they’re qualified to be called ‘skilled in the elements’.
ettāvatāpi kho, ānanda, ‘dhātukusalo bhikkhū’ti alaṃvacanāyā”ti.
The Pali suttas use the term "sa dhatu" rather than "sabhava".
Mendicants, whether Realized Ones arise or not, this element
persists, this regularity of natural principles, this invariance of
Uppādā vā, bhikkhave, tathāgatānaṃ anuppādā vā tathāgatānaṃ, ṭhitāva
sā dhātu dhammaṭṭhitatā dhammaniyāmatā.
The term "sabhava" has a valid meaning in Theravada Commentary but because the term contains the word "bhava" (which has different meanings & can mean "ego"), the Mahayana misconstrue "sabhava" and use for for their propaganda. Also, the Mahayana teaches emptiness different to the Buddha. Mahayana exists because people have different ideas. In the Pali suttas, the term "emptiness" does not mean "empty of inherent existence". It means empty of "ego" or "I-thinking" & "my-thinking". Theravada does not concern itself with whether "things" exist or not. Theravada only concerns itself with ending suffering; which occurs with the ending of craving, attachment & egoism. If I believe a computer exists, this does not cause suffering. Suffering only occurs when I regard a computer as "my computer".