ubiquitin,
The people Joseph knows are perhaps being machmir in kashrus but in so doing are being veeeery meikel regarding the din of eid echad neeman, and giving people chezkas kashrus.
I think that would be highly dependent on the reason a person has for not “mishing.”
For example: Reuven can think Shimon keeps a scrupulously kosher home, but doesn’t think that Shimon keeps his X or Y chumra. X and Y are important to Reuven, so he eats his own food. I don’t think that is failing to give Shimon a chezkas kashrus.
Just for the record, I “mish.”