If the 0.4% number is low, it isn't far off.
https://international.la-croix.com/news/evangelicals-in-france-continue-to-grow-in-number/6155#
Thus, while 2010 statistics indicated that Protestants comprise 2.5 to 2.8% of the French population, the 2017 survey shows that they now number a little over 3%.
Take into account that not all Protestant mainline demoninations are evangelical. Some, like here in the US, are off the Biblical reservation (such as United Church of Christ, Presbyterian USA, Progressive National Baptists, etc.) The percentage of the population of France that are evangelical, Biblical Christians in France is still very, very, very low.
Wow! What a scrambled mess. One of my favorite courses in college over a half century ago was affectionately called by the students, “How to Lie with Statistics.”
I shall not go into the convoluted assertions made by the article as well as your very clear statement that “Bible believing Christians in France is estimated to be 0.4%” other than to say that all statements ignore the history of France and the classifications are misleading.
“Bible Believing Christians” is a total misnomer when ordinary Protestants as well as Catholics are ignored.
France was once known as The First Daughter of the Church. Avignon, France, not Rome, was the papal home for a number of years. French Popes reigned over the Catholic Church.
The outright war between Catholics and Protestant(Huguenots)
lasted quite a while. The Huguenots fled the country due to persecution and settled in many states along the eastern seaboard including Massachusetts, New York, Pennsylvania, Virginia, South Carolina and also in Canada.
If the 0.4 % designates Evangelicals, that s possible, but to broadly eliminate Catholics and most Protestants as not being “Bible believing Christians” is ridiculous, indeed outrageous.
I suggest that statement be clarified.