I was bumping into the word them used together with mass nouns in some translations from Chinese, but according to this ELL post, we don't use plurals with mass nouns.
And now I encounter the 10k medium article which has the same behavior, and I am seriously concerned that I miss something.
Maximum likelihood estimation is a method that determines values for the parameters of a model. The parameter values are found such that they maximize the likelihood that the process described by the model produced the data that were actually observed.
Perhaps the author wanted to say something like data points that were observed? Otherwise I don't understand where it is appropriate to use plural verbs and pronouns with mass/uncountable nouns and where not.