I was not attempting to change a legal definition. Maybe instead of 'arguably' I should have written "it could be argued that". This does not mean that I was making that argument.
The lack of internal borders make it very difficult for France or Germany to identify the first place of landfall for these migrants. This is a problem of the EU's own making. UK kept out of Schengen, and so retains an ability to attempt to control borders. We can see (in most cases) where those crossing the channel started from, and so should be able to return them.