No they are not synonymous.
UPSERT can be implemented without using the
MERGE keyword and conversely
UPSERT is just one possible use case for
MERGE (when it has both a
WHEN MATCHED THEN UPDATE and
WHEN NOT MATCHED [ BY TARGET ] THEN INSERT ...)
At least in SQL Server it is not required that
MERGE have both (or indeed either) of these clauses.
The following use of
MERGE neither updates nor inserts.
CREATE TABLE T(A INT)
INSERT INTO T VALUES (1),(2),(3);
MERGE INTO T
USING (VALUES (1),
ON T.A = S.A
WHEN NOT MATCHED BY SOURCE THEN
Other (possibly SQL Server centric) use cases for Merge that have nothing to do with Upserts is inserting
DEFAULT VALUES for multiple rows or Using merge..output to get mapping between source.id and target.id