Your hypothetical player thinks it's plausible and fits with the campaign setting as constructed, otherwise they wouldn't have used the ability, right?
My hypothetical player knows how I handle background features because I discussed such things in my session 0. If I say it doesn't apply here and they insist I'll still tell them no. If they argue about it then hypothetically they will no longer be a player in my campaign and I wish them luck in finding a DM that works for them because obviously I'm not a good fit. I can't be the right DM for everyone and I don't pretend otherwise.