Not able to produce this with PG 11.1
If col1 is any type other than boolean, the update statement fails in syntax.
If col1 is boolean, then it updated it correctly. In other words
update col1 = NULL
and col2 in (1,2)
is treated same as
update col1 = NULL
where col2 in (1,2)
Also I checked it on DB2 which rejected the UPDATE sql as bad syntax, as expected.