I don't know anything about Spring, but the equivalent technique in an ordinary Java PreparedStatement doesn't do what you want either. So I wouldn't be at all surprised to find that it doesn't do what you want in Spring.
I had read that Spring does "extend" the basic JDBC Prepared Statement capabilities (much like Spring's JDBCTemplate extends JDBC) to actually allow multiple values in a "in (?)" to be added programmatically via a List or HashMap.
The code sample I posted compiles fine as well which leads me to believe it should be possible. However, with much I find in Spring, actually getting seemingly simple things to work as expected can take much longer than expected.
In any case, the very easy and (eventually) obvious workaround I created (avoiding Spring's complexity entirely) is this:
1. Create a String variable, say stringVar.
2. Programmatically build this string with the values for the SQL "IN" clause while parsing the input Request received.
So, stringVar might have values like this: "(1,2,5,6)".
3. On the SQL statement's string variable with the "IN" clause, do a simple sql.replace("?", stringVar).
4. Limit the number of values in the "IN" clause to be less than 100 per the JDBC sepc.
5. Execute the SQL statement however.
Works great. No Spring needed. No Spring error stacks. No confusion. Done.