Not sure what you actually mean when you say that SQL NULL means unknown but not absent? Is there a meaningful distinction you are making here?
It makes a difference when you start applying operations.
For example, if you compare a NULL to any value (even another NULL), the result is also NULL, rather than TRUE or FALSE. This doesn't make sense for absent values - two absent values should compare equal (and, indeed, two nulls in JS do). On the other hand, it makes perfect sense if NULL means unknown - if my last name is unknown, and your lastname is unknown, comparing them for equality can only produce "unknown" as a result, since it's not known whether they're the same or different.
Same thing with arithmetic operations. 1 + NULL equals NULL in SQL, again, because NULL is really "unknown", and so when you add an unknown value to 1, the result is also unknown. If NULL were an absent value, the expression should either produce an error, or give 1.
The most telling part, though, is the SQL truth table for Boolean operators that includes NULLs. Specifically:
TRUE AND NULL = NULL
FALSE AND NULL = FALSE
TRUE OR NULL = TRUE
FALSE OR NULL = NULL
Again, this makes perfect sense if and only if NULL means unknown. AND is always false if one of the operands is guaranteed to be false, so FALSE AND NULL is always false, regardless of what the actual unknown value is. On the other hand, FALSE AND NULL is NULL, because the result could be either false or true depending on the unknown value. With OR, it's the reverse - TRUE OR NULL is TRUE, because OR is always true if one of the operands is definitely true, regardless of what the other operand is. FALSE OR NULL is NULL because the result depends on the unknown value.
Philosophically, the difference also exists. Absent value means "I know what the value is, and there isn't one". For example, for a guy from Iceland, you know his last name - he doesn't have one. Unknown value means "I don't know what the value is, and there could be one". For example, you don't know if I'm from Iceland or not, so I may or may not have a last name, and you don't know which one if I do. These are two distinct states, and ought to be reflected as such in the database.