If this is an engineer, they probably use whatever tool they're most familiar with, and that tool is probably set up for mostly engineering (code) work, in which case in my experience spell checking is a major distraction with an outsized ratio of false positives, because my identifiers aren't chosen for their validity in a wordlist. Even in markdown, it's more annoying than helpful most of the time, so I tend to leave it as a manual step that I mostly ignore. People know what I mean 99.99% of the time. So, that'd be my guess. Any sort of editor that's been tuned to an engineer's preferences which includes only manual spell checking.