Question, where do you live? In America, and especially on the Pacific Coast 'Have a great day' is a phatic expression [1]. It is a common parting phrase and has lost any special significance that it might have once had within a conversation.

[1] https://en.wikipedia.org/wiki/Phatic_expression

TIL phatic expression

and ngl yes of course I read it as something else first.

Anyway maybe they were glad that someone threw them even a phatic expression at all? :)