It is an old tale for people who were quite ignorant and could believe that…
God spoke to very, very ignorant people in 2000 before Christ. So the message was for them but to be read today thinking it was intended for them.
The Council of Trent enumerates Genesis among the books that are the word of God - who cannot make mistakes and who cannot lie. It also makes clear it must be understood according to unanimous interpretation of Church Fathers (i e presumably whenever they are unanimous, their interpretation obliges, when they are not at least their common ground obliges).
Which exact Church Father supports that view, and how is it made clear in Genesis that the old ages come in a passage to be treated as a quote?
Now, first off, to speak seriously, whether God speaks to ignorant or to wise, he is no liar.
Second off, whatever the ignorance of any human society, it was certainly not ignorant of the ordinary lifespan of man.
Third off, the high ages being original and the considerably lower ones we have now, it is those who need explanation: and they are explained as God deciding to shorten human lifespans after the flood.
Which would be the explanation needed.
My favorite theory to be honest is that mankind was in the process of falling further and further, and that his original DNA was better than it is today. (So they lived longer, because they were closer to the original man in his created glory.)
I have recently read that man has a gene for producing vitamin C, but it is turned off, i e men cannot produce vitamin C in their own body. That may be part of the DNA degradation involved. However, I think it happened suddenly, more or less, on a divine decision.