About the discussion of the relationship of jhāna and samma-samādhi:
“Samma” – one of it’s meanings is “tuned”, as in musical instruments. All 8 factors, correctly tuned together and established, constitute the Path that Buddha taught.
On the other hand, that the Buddha used jhāna as samma-samādhi doesn’t mean jhāna can’t be used for other purposes. (To perhaps echo a point made by others here.) There’s good evidence that Vedic/Brahmanic practitioners used it, but sans the integration in a path insight directed at achieving mental liberation in this life, i.e. they used it more or less as temporary escape from suffering, along a path which was fulfilled in post-death merger with some Self-Brahma sort of thing. At least that’s the picture I get from some study of that tradition.
Jhāna is, in Sanskrit, dhyāna, which most generally means sitting still there “meditating”. It would seem perfectly plausible that anyone doing that extensively could happen across full absorption, which is, after all, a distinctive mental capability that can be trained as a skill, independently of any particular religious dhamma system. Using it to hone the mind for penetrative insight into the nature of phenomenal experience (deconstruction of anicca, dukkha, anatta etc.), in integrated partnership with sila and paññā, is another matter, which what distinguishes the Buddha’s discovery.
And vis-a-vis the discussion as to whether sensory stimuli are perceived in jhāna or not, it depends on what’s meant by perception. I find it quite plausible that one can, at least at the earlier (1st to 3rd) stages, be “aware” in some sense that external stimuli are present (the bare mind-moments of passive contact), but the mind, being unified, stilled (some say “frozen”, but that’s a bit extreme), doesn’t react, doesn’t engage and further interact with those stimuli; it’s as if they bounce-off a sort of invisible shell surrounding the absorbed mind. With deeper absorption (4th stage and beyond), the mind is beyond reach of any awareness of contact with external stimuli. Possibly excepting relatively violent intrusions that can knock the mind out of absorption, and for one maybe not having fully mastered the skill.
(Relatively deep psychedelic experience offers a comparison here. There the mind can become artificially disabled from higher-order cognitive processing of sensory stimuli; a forced sort of absorption into more primitive layers of consciousness. Colors, shapes, sounds etc. are there but the mind is incapable of integrating them with memories, associations, concepts, etc. One doesn’t “hear” music, or “see” a painting in the conventional sense, but only fascinating (but otherwise meaningless) swirls of visual and auditory effects. This doesn’t involve skill or understanding, per se, but can lead, upon later reflection, to insight into the distinction between the nervous system functions of receiving sensory stimuli and the activities of the (intact) mental powers of higher cognition; can open the door to realizing the difference between pure phenomena (bare appearance) and “reality” (their meaning as supplied by the mind). )