Signless is not a jhana and is not neither perception nor non perception. For example, both SN 40.9 & MN 121 appear to say the signless can be attained after the sphere of neither perception nor non perception.
However, other suttas, such as SN 41.6, repeated in MN 44, say the signless can be attained after emerging from the cessation of perception & feeling.
Therefore, the impression is the foremost signless does not occur directly from a furtherance of "tranquilization" (as described in SN 36.11, MN 111, MN 26, etc) after the sphere of neither perception nor non perception. Instead, it appears the signless occurs after emerging from the sphere of neither perception nor non perception; or otherwise, emerging from the cessation of perception & feeling; when the mind becomes increasing conscious rather than less conscious.
In conclusion, MN 43 and particularly MN 121 appears to clearly say the signless is not the foremost in liberation; is not emptiness (sunnata); and is not Nibbana. MN 121 says:
‘Even this signless immersion of the heart is produced by choices and intentions.’
‘ayampi kho animitto cetosamādhi abhisaṅkhato abhisañcetayito’.
They understand: ‘But whatever is produced by choices and intentions is impermanent and liable to cessation.’
‘Yaṃ kho pana kiñci abhisaṅkhataṃ abhisañcetayitaṃ tadaniccaṃ nirodhadhamman’ti pajānāti.
Therefore, when the Mahayana attempts to say the signless is superior to Nibbana; that Nibbana is merely a sign or concept; that Nibbana is the same as Samsara; obviously the Mahayana are wrong.