Others are also affected by islands and territories, see France with French Guyana etc, Norway with Svalbard etc. France is quite a square country if you only consider its mainland so it is an odd choice to include non mainland areas but I guess it is a difficult greyzone on what to include and not.

The French themselves refer to metropolitan France as l'Hexagone so it probably still would not be a square.

Somehow I was thinking about France with French Guyana but must have passed it.

As some have said, I'd like to check the impact of only using mainland in the calculations.

I'd say France is far more a pentagon than a rectangle.