They werent saying "our own" (in the sense) of making it up for themselves (on the spot to Jeremiah) but as what was passed down to them. It shows right (in the verse) they are stating
AS WE HAVE done
& OUR fathers, princes etc etc. Its an indicator (set forth as an example) as to how they answered the prophet of God (Jeremiah) by whom God did speak. These (there) are showing themselves as an example of turning their ears from the truth (in that sense). He told them to not do that. But they would answer
our own mouth (and traditionally so). Right there is a perfect example of how error is also traditionally taught (and orally so). The prophets (even Jeremiah in this case) is as a confronter to their example (which can be that which we learn by).
1Cr 10:6
Now these things were our examples,
to the intent we should not lust after evil things, as they also lusted.
And a true prophet (which Jeremiah was) shows himself forth in this same wonderful example...
Jerm 28:6
Even the prophet Jeremiah said, Amen:
the LORD do so:
the LORD perform thy words which thou hast prophesied, to bring again the vessels of the LORD'S house, and all that is carried away captive, from Babylon into this place.
Jerm 28:7
Nevertheless hear thou now this word that I speak in thine ears, and in the ears of all the people;
Jerm 28:8
The prophets that have been before me and before thee of old prophesied both against many countries, and against great kingdoms, of war, and of evil, and of pestilence.
Just like the prophets of old, they would be true to the words of the former prophets just as those who followed after the apostles should be true to the apostes. Error is standing corrected by the prophet according to the former prophets,
notice the patern for us?
1Cr 10:6
Now these things were our examples