Actually, I do not think that it is "extremely clear". The legislation limiting movements to specific purposes is silent regarding its application to international movements so it is a question of interpretation whether it applies to them or not. If you read it literally and assume that it applies to international movements, then it would apply to inbound movements as well as outbound movements. It would mean that a French national, for instance, could only enter France for one of the specified reasons as listed on the attestation derogatoire.
I don't think that this reading is correct. The rules on confinement are not designed to apply to international movements. They are meant to restrict movements within France to limit the spread of the virus on the territory. There are covid-related rules regarding international movement but this is a distinct set of rules with its own, distinct system of attestation derogatoire (from which (inter alia) French nationals and intra-EU movements are exempted).

Now, you could argue that perhaps the lockdown rules do not cover international travel but they in any event prevent all domestic travel and therefore travel to the airport from your home and that, even if you were allowed to go abroad, you are not allowed to go to the airport unless for one of the reasons listed in the attestation derogatoire. This is true on a literal reading of the legislation but, if so, it is also true for inbound as well as outbounds movements. So, it would mean that somebody whose entry to France is otherwise authorised would, on arrival in France, have to remain at the airport until the lockdown is lifted or turn back to their country of origin (or another country willing to accept them). I don't think that this is a correct reading either. I would have thought that an exception for domestic travel linked to international travel must also be read in the legislation.

In truth, it seems to me that the question of international travel did not even enter the mind of the legislator when drafting the lockdown rules. They are simply not designed with international travel in mind and I cannot find any official document that suggests that the lockdown rules apply to international travel. Now, whether you'd be able to have a meaningful discussion on this with your average cop stopping you on your way to the airport is another matter but if you do reach the airport, I would be extremely surprised if you were stopped at the border.by a DCPAF officer. AIUI, the DCPAF applies rules relating to international travel (including restrictions on entry for individuals coming from non-EU countries from which movement is restricted on covid grounds) but do not concern themselves with lockdown rules as such. Some of the contributors on this board, including highly respected ones, think that this is not the case. Do we have evidence for this or is this just an assumption?

FWIW, I traveled from abroad (intra-EU) yesterday (i.e. after the "tolerance" period), including traveling about 150 kms by public transport (train and bus) between the airport and my home in France. My passport was checked on entry into France but no question was asked regarding the purpose of my travel nor my final destination, etc...

When traveling between the airport and my home, I was not asked any question either so I cannot tell what would have happened had I been asked for an attestation. There were police officers at one of the stations but they were just patrolling normally and were not checking attestations. They did ask one person who was not wearing their face mask properly to put it right but that was about it in terms of covid rules enforcement.