I wonder what percentage of their employees have children, or come from poverty, or come from rural communities, or speak a language other than English at home, or have a criminal record, or lack formal education. Maybe we could see some age brackets too, or a distribution of political affiliations/views. Won't hold my breath for those numbers, as those are not considered components of "Diversity," but it would make it easier to take the last bit seriously:

We have always said we want Slack to be a place where people of all different backgrounds thrive.

There has been a historical lack of representation for women, people of colour, and LGBTQ people in the workplace. Some of the social strata you point to are covered in these brackets, and some aren't. Certainly it would be nice to see a company indicating that they take concerns about ageism seriously, along with providing support to ESL persons and/or indicating that their hiring process doesn't unfairly exclude people without a post-secondary education, but I'm not sure coming from a rural background or having a particular political affiliation is correlated with employment discrimination or exclusion in the workplace (with the obvious caveats for being a neo-Nazi or a jihadist or something).

In the aggregate, though, as a visible minority and a political 'ugly duckling' in my current workplace, I like that companies are being somewhat transparent about how they support diversity. I would imagine they'd welcome feedback such as yours.