It seems to me that a nested sets tree must have a single root node. Is
this true?

I am converting an Adjacency List hierarchy to Nested Sets. In a situation
where I am organizing a list of manufacturers into a tree, there is no
single root manufacturer - so I would have to create an "artificial" root
node called "All Manufacturers", correct?

Cathead

Yes; that is the mathemtical definition of a tree and a root node; it
would not matter how you modeled it.

there is no single root manufacturer - so I would have to create an
"artificial" root node called "All Manufacturers", correct? <<

That would work, or you could go with a forest -- a table of trees

CREATE TABLE Forest
(tree_nbr INTEGER NOT NULL,
node CHAR(10) NOT NULL,
lft INTEGER NOT NULL,
rgt INTEGER NOT NULL,
CHECK (...),
PRIMARY KEY (tree_nbr, lft, rgt));