But we do not have that kind of context. We only have a assumed intention on what the person meant by it. If we do such assumptions, we assume bad faith.
It could be that they hired a new web developer, also called web designers by many, or designers as short. If a back-end or sysadmin meet someone who is working on the front end, or even on a API structure, its fully plausible to call them "designers", be them male, female, or alien.
Simply put, the comment without any additional context to vilify the speaker, is not sexism. With some context, it might be, but in many others, it is not. Assuming good faith is to default at those context that do not mean sexism.
I.e., suppose a new girl shows up at your office. Someone is offering even money that she is a developer. Do you take the bet?
(tl;dr: It's almost certainly an accurate deduction, but acting on it might make society/culture/company worse off, so let's not do it)