The approach seemed to be, if things break, people will report it and we’ll fix it.
While this may not be the best approach, the number of languages supported is too high for a person to check each one manually. Generally, I imagine they wouldn't expect a change like this to break anything significant.
[people] use it as a portfolio. [..] To suddenly doink the appearance of people’s portfolios is unfortunate.
It is very unlikely that syntax highlighting errors in GitHub will affect someone's chances of getting a job.
Sure, this switch could cause some issues but they don't seem to be severe enough to kick up a fuss over.