The only reason I believe that those posts were not written by the same person are the dates - the time between Satoshi's post and Peter Todd's post. It's one day apart. If it was a continuation of Satoshi's thoughts posted mistakenly using another account, then it would have been mere minutes apart, no?
There are more reasons than that. For example: Satoshi really thought about a model, where all old inputs and outputs are kept, as they were, and where new inputs and outputs can be added, to increase the fee.
For example: you can have this transaction:
+------------------------------------------+
| Alice 50.00 BTC -> Bob 10.00 BTC |
| Charlie 40.00 BTC |
+------------------------------------------+
And then, Peter Todd thought about this case:
+------------------------------------------+
| Alice 50.00 BTC -> Bob 10.00 BTC |
| Charlie 39.99 BTC |
+------------------------------------------+
But note, that in this case, you can lose some anonymity, by revealing, which output has less coins than before, and which coin is a potential change (or: you would then force a recipient to pay for increased fees, which is a bad idea).
Instead, what Satoshi thought about, was something like this:
+------------------------------------------+
| Alice 50.00 BTC -> Bob 10.00 BTC |
| Fees 0.01 BTC Charlie 40.00 BTC |
+------------------------------------------+
Note that in the latest version, you can actually do that, if you use anchors, for example:
https://mempool.space/testnet4/address/tb1pfees9rn5nz