The OP hasn't actually mentioned having issues with SNCF. Rather she is being a sensible and prudent traveller in my opinion in just altering her itinerary to an alternative and open route.
She asked quite a narrow and reasonable practical question.
Doing what I would do in this position, just get on with it and re-plan, not shouting demanding my "rights".
The law, as stated by Wengen (and I assume it is right) is not designed or really intended to apply in a case like this where there is a major, unplanned line blockage. From the posts I have seen there doesn't even seem to be the option of providing alternative road transport over the affected section as the highway also seems to be closed.
Letter of the law and intent of the law are quite different things- not just in this transport related case. In the case given by @Bets that seems to be what this quoted law was meant to be about.
I, for one, do not think that SNCF are being unreasonable in this instance.
There are probably sensible reasons why SNCF can not divert the international trains to Milan via Ventimiglia- like line capacity and driver route knowledge and timing constraints.
On the narrow question- given the SNCF frequency into Ventimiglia I would have assumed they have their own TVM's there, but the SNCF connect website says physical tickets can be collected at any station. So collecting at Lyon just sounds like a sensible prudent option to me.