"Koe" is a de-word, hence the use of "deze".
The "het" in this sentence is a quirk of the predicate superlative (i.e. a superlative that is connected to its noun by a linking verb, like "to be"). In that case you can use either "de" or "het", regardless of the noun. They have slightly different meanings:
- Deze koe is het zachtst - This cow is the softest
- Deze koe is de zachtste - This cow is the softest one.