Yeah, but who ever writes "x=0.9" as a constraint on a partial index? Really? Don't you know you aren't suppose to compare floating point quantities for equality?
If P is the expression on the partial index and Q is the WHERE clause of the query, then the partial index is only usable if Q implies P for all possible assignments of variables. A theorem prover is needed to establish this. Every RDBMS has one. The one inside SQLite is not terribly bright, true enough. It leans toward usually little memory and few CPU cycles. It does not do a good job if P contains "x=0.9". On the other hand, SQLite's theorem prover is decent if P contains "x IS NOT NULL", because in actual practice, probably about 90% of partial index WHERE clauses are some variation on "x IS NOT NULL".
The partial index expression does not always have to be exactly the same as what is in the WHERE clause of the query. SQLite will always find the match if P is a subset of Q; if Q can be rewritten as "R AND P". But if P is "x IS NOT NULL" and Q does anything that restricts x from being NULL, for example if Q contains "x>0", then SQLite's theorem prover will find that match too, even if "IS NOT NULL" never appears in Q.
Will the theorem prover in SQLite get better someday? Perhaps. It has gotten better over the years. The question becomes, how much more code space and query-planning CPU cycles are you willing to spend to get a slightly better query planner? This trade-off is different for a client/server database engine. With SQLite being embedded, the trade-off tends to fall more on the side of "keep it simple". If you have followed SQLite over many years, you might have noticed it is shifting toward more complex decision making as memory becomes cheaper and CPUs get faster. It's a tricky balancing act to find the sweet spot.