That psychology professor at UC Berkeley is misinformed on the subject.
Datum is the singular form.
Data is the plural form of datum.
Check Webster, Oxford, etc.
A similar example would be "Media" and "Medium"; Medium is the singular form of Media.
Data can be used as a singular non-countable noun nowadays, but the latin origin of "Data" is "Datum".
Nobody uses "Datas", or worse, "Datae".
Now, on the pronunciation...
Before even thinking about how to pronounce "Data", think about how you would pronounce "Datum".
If you start the first syllable of Media and Medium the same way, wouldn't you do the same for first syllable of Data and Datum?
Here's a little rote to help: "Orlando Bloom is cute, but I would never 'Datum'."