The plural forms that you are giving are the same ones that I was “hearing” when you gave your original examples, so I think that the literal interpretation of ninnu/ninw as the demonstrative (“these/those”) still can hold.
That said, your teacher/friend/interlocutor might be referring to or thinking that the proper interpretation (of at least some of the examples) is in fact with the diminutive suffix -nin (equivalent to ‘wee’ or ‘little’ in English, but it gets tacked onto the end of a word; it is the counterpart of -ba).
In this case, one of your examples could be the following (I’m going to put in dashes for clarity’s sake since in this case, “sufɛkumaninw” would actually be one word tonally):
N ye n miiri an ka su-fɛ-kuma-nin-w na
‘I thought of our little words of the night’
(Note that sufɛkumaninw could likely be pronounced more like [sufɛkumaninnu], but I am not writing it here since -lu/-nu isn’t normally considered “standard” Latin-based Bambara orthography. It does show up in dictionaries though and in N’ko orthography, -lu/nu is actually the standard.)
Tonally there would normally be a clear difference between this:
Kumaninw (or, kumaninnu)
In Latin-based Bambara, there isn’t a standard way of marking this, but the short of it is -nin is a suffix without its own tone (it gets determined by the word it attaches to), whereas nin (‘this/that’) is a low tone word. So, simplifying things (since tone marking is complicated and can quickly involve various theories and assumptions of what is the underlying tone vs the surface realization that one hears), we can “see” the difference like this (an acute accent like this one <é> means high tone and a grave accent like this <è> means low tone) :
All that to say that -nin does indeed mean “small” just like dɔɔnin or fitinin and it can carry the plural marker (ninw or ninnu). And when plural and spoken at a decent pace, one might hear it is [nu].
But the question of whether that’s the right interpretation (instead of the demonstrative nin) for a particular sentence is another matter! In that case, comparing how someone pronounces the tones of the sentence could be helpful.