This is actually not true. Sakkayaditthi is defined 4-fold:
- Khandha(s) as self
- Self as in khandhas
- Khandhas as in self
- Self as having khandhas
If the khandhas are within the citta that is outside of them, or the citta is the owner / controller of the khandhas, both of which are claimed, then it is sakkāyaditthi. Taking the khandhas purely as self is pretty much the annihilationist view, not the eternalist view, because someone can see that the khandhas are impermanent and non-self but still has ideas of a larger self outside or inside them.
I would also point to SN 12.15 and SN 22.90, or for instance MN 1. Here, conceiving oneself as being anywhere in relation to the all (i.e. anything in experience) is a form of self-view. The sotāpanna knows it’s a delusion, the puthujjana does not. If someone is still seeing something external in relation to the khandhas / experience, it is maññanā as per MN 1. For example, perceiving the khandhas, and perceiving one’s citta as outside (in/from/on) them. Similarly, as per SN 12.15, it perceives ‘the All’ (or samsāra itself) as a truly existing absolute: everything is here, and Nibbāna is a special consciousness outside of the All. The sotāpanna is said to know the arising and ceasing of ‘the All,’ ‘the world,’ etc. and that this knowledge is necessary for sotapatti. If one thinks that there is ‘the world’ and a separate outside reality that exists separate from that, then one is stuck in self-view/conceiving and reifying what is actually dependently arisen experience.
Also, sotapatti is required for any further attainment, including non-return. I would perhaps say that there is a difference between freedom from sensuality and non-returnership. There seem to have been ascetics and mentions of them in the time of the Buddha that were generally free of sensuality temporarily, but they would be reborn in high Brahma / formless abodes and would eventually fall back. So in a sense, their kāmatanhā was suppressed, and they only had bhavatanhā for form/formless realms, similar to the anāgāmi. In actuality though, it was only temporarily suppressed for this lifetime. It seems this was actually the dominant way very highly attained spiritual ascetics were viewed at the time by Buddhists: they had jhānas or formless attainments and would be reborn there rather than in sensual realms, but it was not unshakable / permanent, unlike non-returnership and the Pure Abodes.
The idea of a pure consciousness and the Citta as formless is basically just one of the views of self around at the time that’s somewhat subtle, of a formless (perception-less?) self, perhaps. I’m not entirely sure. But this basically seems to be a case of someone letting go of all worldly attachments and taking refuge in a kind of formless-realm 24/7, which is and was possible even without non-returnership; it’s just temporary. Anything that happens physically one has let go of, their mind established in that formless idea.