Furthermore, it's not unreasonable for people living in one country to have opinions on the policies of others, especially when it's something like taking in refugees which has strong influences on a global scale.
And in this specific case: citizenship and residency ought to give one's voice more weight.
When members of a group discuss policy for that group, they do so under the implicit assumption that their values are similar enough that what they are doing in the conversation is trying to find a way to achieve their shared values.
But the great-great-great-grandparent comment was simply a statement of values, not a suggestion of how parties concerned could achieve theirs.
I realize the lines are fuzzy: HN is certainly not a 'murican forum for 'murican issues. But its composition is such that neither is it a non-American forum. As such, the greatx3-grandparent could have been more sensitive.
That is ad hominem -- specifically, ad hominem circumstantial [0] -- and lies at the nexus between the genetic fallacy and appeal to motive.