select dblink_connect('myconn', 'dbname=mybase');

insert into marks (mk_guid, mk_st_guid, mk_mark5)
select mk_guid, mk_st_guid, mk_mark5
from dblink('myconn','select mk_guid, mk_st_guid, mk_mark5 from marks limit 100')
as t (mk_guid character(36), mk_st_guid character varying(36), mk_mark5 integer)
on conflict (mk_guid) do nothing ;
INSERT 0 100

select dblink_connect('myconn', 'dbname=mybase');

insert into marks (mk_guid, mk_st_guid, mk_mark5)
  select mk_guid, mk_st_guid, mk_mark5
    from dblink('myconn',
      'select mk_guid, mk_st_guid, mk_mark5 from marks limit 100')
  as t (mk_guid character(36),
         mk_st_guid character varying(36),
         mk_mark5 integer)
  on conflict (mk_guid) do update set
    marks.mk_guid = t.mk_guid,
    marks.mk_st_guid = t.mk_st_guid,
    marks.mk_mark5 = t.mk_mark5;

ERROR:  missing FROM-clause entry for table "t"
LINE 9:     marks.mk_guid = t.mk_guid, 
                            ^