Not unrelated to a longstanding policy of suppressing regional languages:

> Depuis plus de deux siècles, les pouvoirs politiques ont combattu les langues régionales

https://en.m.wikipedia.org/wiki/Languages_of_France

Years ago, I went to live and work in Strasbourg. My French was… rudimentary, school-level, but after a few weeks I was picking up the rhythm and following along. Then the grand chef came up from Paris. During the night out entertaining him, I asked him to slow down a bit as I was struggling with his accent. He completely lost it, insulting the locals as peasants, and claiming the accent was theirs not his. Kind of put a damper on the evening.

Obviously the Marseille “dialect” is recognisable, but otherwise, travelling throughout France, and even the French-speaking parts of Switzerland, I could understand folk.

What about Ch'ti [1] or Savoyard?

The article is about dialects not accents. Even just considering French accents, I find the Marseille one distinctive.

[1] https://en.wikipedia.org/wiki/Picard_language