orthostatic hypotension is describing a consequence, a symptom, it is basically as you say, a decrease in blood pressure due to a change in position
it is NOT just caused by autonomic dysfunction
it can have different ultimate causes, such as autonomic dysfunction, blood loss, dehydration, etc
this would obviously make a difference in tx
ALSO
orthostatic hypotension, or any of its causes, may or may NOT lead to syncope
people may use orthostatic hypotension or dehydration interchangeably in discussing syncope because dehydration --> orthostatic hypotension --> syncope
essentially, that is how dehydration LEADS to syncope, by leading to a decrease in blood pressure 2/2 change in position
syncope itself describes a symptom, it can have different causes, some of which do NOT include orthostatic hypotension
I hope this helps you to think about orthostatic hypotension and syncope, they are not synonymous and they can have different proximal causes
all of that said, I would say that if you are discussing orthostatic hypotension NOT 2/2 something besides autonomic dysfunction, then essentially it is implied that is the cause, but as you point out, one would need to actually make that diagnosis by testing
also, you would only use the tilt table test to rule in or rule out autonomic dysfunction in orthostatic hypotension or syncope
typically before you get to tilt table you have already dealt with more obvious and easily treated potential causes such as dehydration
as you more precisely identify the cause of syncope, you start to use different terminology
AAFP articles on orthostatic hypotension
Orthostatic Hypotension - American Family Physician
Evaluation and Management of Orthostatic Hypotension - American Family Physician
SHORT ANSWER:
Actually, dehydration, blood loss, hypovolemia 2/2 any cause can be a cause of orthostatic hypotension
Once you have identified the cause of orthostatic hypotension, it would be silly to simply say orthostatic hypotension is the cause of syncope
In medicine, once you are able to describe something more precisely, you should
Orthostatic hypotension NOT secondary to causes besides autonomic dysfunction, usually implies that as the cause
In practice it would be silly to use tilt table to rule in or out dehydration syncope, one would just first treat any dehydration, if then the patient no longer has orthostatic hypotension, there is no need for the test, which is more expensive and time consuming than an IV bag or water, and more uncomfortable too