(My understanding of this tweet is my own. Tweets are too short to fully understand the message) In the tweet I do read Understanding is definitively subjective and is not measurable. (I leave <holistically> out, because I do not Know what holistically means in this tweet)
Questions: Is a subjective entity not measurable? Yes it is measurable. “Do you like peanut butter?” Your answer is a measurement: Yes, no, i like it very much, I love it, i hate it. It is measurement.
Is it exact and countable measurement? That is another question.
Is understanding subjective? What do you mean with <subjective>? Do you mean Subjectivity, a subject’s personal perspective, feelings, beliefs, desires or discovery, as opposed to those made from an independent, objective, point of view?
To know if something like <understanding> is measurable we must describe what we mean with this word in a situation.
Understanding (also called intellection) is a psychological process related to an abstract or physical object, such as a person, situation, or message whereby one is able to think about it and use concepts to deal adequately with that object. Understanding is a relation between the knower and an object of understanding. Understanding implies abilities and dispositions with respect to an object of knowledge sufficient to support intelligent behavior. (Bereiter, Carl. “Education and mind in the Knowledge Age”.)
So if a person has some abilities or is able to think about it than we know there is understanding. (U = understanding; U > 0 ) We could describe some abilities that show understanding of a given object. And we could use this description to measure understanding in a context or a situation.
Measurement is not the same as counting.