the vibe I get is someone who can't put in the effort to make my job reading easier (i.e. hard to find sentence breaks)

It is on a human seeing level, harder to parse. If they don't want to use proper grammar and punctuation, it reflects on their seriousness and how serious I should take their writing (not at all because I'm not going to read difficult to parse text) The same goes for choosing bad fonts or colors that don't contrast enough