IS NOT DISTINCT FROM statement

I have some query:

EXPLAIN ANALYZE select id from sometable where fkey IS NOT DISTINCT FROM
21580;

QUERY PLAN

----------------------------------------------------------------------------
------------------------------------------------

Gather (cost=10.00..39465.11 rows=1 width=4) (actual time=0.512..129.625
rows=1 loops=1)

Workers Planned: 4

Workers Launched: 4

-> Parallel Seq Scan on sometable (cost=0.00..39455.01 rows=1 width=4)
(actual time=77.995..103.806 rows=0 loops=5)

Filter: (NOT (fkey IS DISTINCT FROM 21580))

Rows Removed by Filter: 675238

Planning time: 0.101 ms

Execution time: 148.517 ms

Other Query:

EXPLAIN ANALYZE select id from table where fkey=21580;

QUERY PLAN

----------------------------------------------------------------------------
------------------------------------------------------

Index Scan using sometable_index1 on sometable (cost=0.43..8.45 rows=1
width=4) (actual time=0.075..0.076 rows=1 loops=1)

Index Cond: (fkey = 21580)

Planning time: 0.117 ms

Execution time: 0.101 ms

(4 rows)

There is unique index on sometable(fkey);

Is there any reason that "NOT DISTINCT FROM" can't be autotransformed to "="
when value on right side of expression is not NULL or is this any way to use
index with "IS NOT DISTINCT FROM" statement?

Artur Zajac