The context is different though - the professor is stating that an anecdote is a data point, and plural data points can be referred to as data. Two stories are two datums, pluralised to data.
The "not data" version is saying that anecdotes are in such small numbers and uncontrolled circumstances, that even though they are data points in and of themselves, they're not rigorous enough to draw wider conclusions from. It's a different context.
Interesting, but like the author of that post I'm going to stick with "not data". Anecdote usually implies personal experience, which is easily confused and misled.
e.g. I know MANY people who claim to have seen a ghost, but I don't consider that useful data supporting the existence of ghosts. Technically it is data, but it's the LEAST trustworthy form of data, hence the dismissive expression.
The issue could easily be bad management, bad hiring, etc etc etc.