I think you are saying that Paul was speaking to believers/saved persons, here, (I agree) but note that he is saying
this to them:
"
1 And I, brothers, was not able to speak to you as to spiritual, but as fleshly—as to infants in Christ.
2 I gave you milk to drink, not solid food, for not yet were you able. In fact, now you are still not able,
3 for you are still fleshly. [...] are you not fleshly,
and are walking according to man?"
[...that is,
as unsaved men do... i.e. the natural man, before salvation; he's not saying they ARE "natural [/unsaved] man," he's saying they are
WALKING "
according to man"]