At the end of this section it is mentioned that:

While the SQL generated in these examples looks the same whether we
invoke select(user_table) or select(User), in the more general case
they do not necessarily render the same thing, as an ORM-mapped class
may be mapped to other kinds of “selectables” besides tables. The
select() that’s against an ORM entity also indicates that ORM-mapped
instances should be returned in a result, which is not the case when
SELECTing from a Table object.

Now I wonder if the following is a bad practice:

stmt = (
    select(User, user_tree_table)
    .join(
        user_tree_table,
        User.id == user_tree_table.c.descendant
    )
)

I could not find any example that mix them together.
By the way, the user_tree_table is a closure table and it is really hard or impossible to use the relationship() here.