Splendid questions, which we could work through…
Here we could already find some complications. If we look only at the SN there is only one sutta which defines sammasamadhi as the four jhanas, namely SN 45.8.
In contrast there are numerous suttas (1) which say
“right concentration, which is based upon seclusion (viveka), dispassion (virāga), and cessation (nirodha), maturing in release (vossagga).” (Bodhi)
“right immersion, which rely on seclusion, fading away, and cessation, and ripen as letting go.” (Sujato)
And while viveka is part of the classic jhana-formula, viraga and nirodha are not. So, if samadhi relies on nirodha then indeed it makes sense to say that it represents the end of the path.
(1) SN 3.18, SN 43.12, SN 45.2, SN 45.9, SN 45.49, SN 45.55, SN 45.63-69, SN 45.77-83, SN 45.91-96, SN 45.127-132, SN 45.139, SN 45.148-162, SN 45.170, SN 45.180
Now, this distribution is also not very satisfying, having basically only the transmitter of SN 45 as the source of this alternative ‘definition’.
The samadhi-soteriology of the Anguttara needs another examination…