Regarding plain language and readability metrics which of the following statements is incorrect?

It is better to write $"1000000"$ rather than $"1$ million"

Flesch Reading Ease and Flesch$-$Kincaid Grade Level metrics are not directly convertible to one another

ISO $24495-1$ is the Plain Language Standard

October $13$th is the International Plain Language Day

Long and convoluted sentences should be avoided as much as possible

Gunning$-$Fog Index has shortcomings when it comes to differentiating between word length and difficulty