This has not been true in my experience, as someone who is used to talking to a mathematician during my academic life (a bit). They prefer to not use words like "just X".

They generally reply in definite statements, "this is undecidable", "this is an example of X", "this can't be done without Y", they rarely say. "X is just Y", they would probably say, "X is Y".

The "just" implies some form of detail that might be missing in the relation generally. Even my mathematical text books of pretty advanced topics rarely used "just".

Again, what you say is most likely true when math people talk amongst themselves but I don't think they do so with other non math people. I was in compsci.