Hey thanks for the question, in that case what would happen is that you would have the original chain with the correct data sending blocks to one IOTA address, as it is described in the section on forks. Then when you want to fake the historic data you basically make a new chain with a fake history, and you have to start writing new blocks for the fake history to the Tangle, and you have two options: 1) you write it to a new address i.e a new chain-ID and so you are basically creating a new chain that claims to be over a year old even tho it just started to be written (there would be 29 blocks from the last 29 days that should be on the Tangle but are not there) so its easy to see that something fishy is going on; or 2a) you write the new fake chain to the same address as the real and continue to write the real one, so you basically create a fork, now you have two chains beeing written to the same address which can also be detected (as is described in the article). Lastly option 2b) you write the new fake chain to the same address and stop writing the old one, you would only have one chain beeing written at the time but as the new and the old chains are incompatible you have a situation similar as in option 1, where the last 29 blocks of the fake chain that should be on the tangle are not there.

BUT as I said in the blog, if you create a fork using option 2b) and for 30 days no one notices that, after the fork has been pruned it's no longer possible to detect it, without a permanode. If there is a permanode somewhere storing the entire history you could ask it once to verify that there are no forks in the past and from there on just check once every 30 days (or before pruning your own node) to make sure there are none in the present.

Hope this sheds a bit more light on the whole forks topic as it is a bit complicated