> In what situation would you walk up to a stranger

Walking up to a stranger is one thing, but that really wasn't the case here... in this case, a comment was made to company-appointed P.R. representative who was present at a meeting with a journalist who was there to write about the company, its values, and its culture. I think the founder of the company commenting on the same company's P.R. person not adhering to the company's values is very much him "minding his own business".

When the stranger works for you and you literally own the business and part of the business's PR strategy is environmentalism and the stranger's job is PR and they're talking to s reporter, I would say it's your business and this is appropriate minding.