I would say that telling someone who emailed you in an unsolicited fashion that you have an expected protocol for interaction isn't in the wrong and putting that line first in an unambiguous way is practically required for how people read emails. They did ask for context after that line, which was fine. I will say that the 'low-effort' part of their phrasing could have been constructed better but the first reply was more in the scope of stern and terse than outright insulting and hostile. I feel like I have to answer dozens of emails like this in a similar fashion because then I get the form letter or PDF that they want to show me over a meeting or they actually give an agenda and I can make the appropriate decision for myself. Since there's an existing relationship here any ambiguity wastes everyone's time, if it was an unrelated entity then you can assume sales email but in this case you can't. Any moderate sized company or larger is a multi-headed hydra so it's never obvious what part of the beast you are talking to and why they are seeking you out.