MERGE behavior with REPEATABLE READ isolation level

Hi hackers,

I found it strange that ExecMergeMatched() checks for
IsolationUsesXactSnapshot() in the TM_Deleted case, but not in the
TM_Updated case.  Indeed, EPQ works on the repeatable read isolation level!

s1# create table test (id int primary key, val int);
s1# insert into test values (1,0);

s2# begin;
s2# update test set val = val + 100;

s1# MERGE INTO test t USING (VALUES (1, 100)) AS s (id, inc)
      ON t.id = s.id
    WHEN MATCHED THEN
      UPDATE SET val = t.val + s.inc
    WHEN NOT MATCHED THEN
      INSERT (id, val) VALUES (s.id, s.inc);
(waiting ...)

s2# commit;
s1# MERGE 1
s1# select * from test;
 id | val
----+-----
  1 | 200
(1 row)

Quick search didn't give an explanation of this to me.  I also don't see
this covered by an isolation test.  Is it an intended behavior or a bug?
(Sorry for buzz if this was discussed before)

------
Regards,
Alexander Korotkov
Supabase