It's a dialect for a proprietary database. Right now, I can share the code below, and I'll look into sharing the whole thing. Also, I used IntelliJ to see what called supportsTuplesInSubqueries, and the only call to it in the 4.3.11.Final source code is in some delete code, so clearly the code that generates the query in the description doesn't call it.

 @Override
    public boolean supportsTupleDistinctCounts() {
        return false; // no support for select count(distinct a, b)
    }

    @Override
    public boolean supportsTuplesInSubqueries() {
        return false; // no support for this.
    }