This is getting down to semantics, but for lack of ED info to discuss.....
The survivor's Curse skill is pretty much designed to be part of a combo. You can use it on its own, but there is no point in doing so unless you are going to follow up with an elemental or poison attack.
I'm using 'combo' here as short for combination, not the specific series of timed attacks used in some combat systems that result in some type of bonus damage, or other forms of stringing individual offensive or defensive moves together.
IMO synergy between skills means inherent connections, not simply skills which have effects that go well together. For example, paralyzing or blinding an opponent and then casting fireballs may be much more effective than simply casting fireballs, but the skills themselves are not interacting, so it is just a combo. If the character learns a firewall spell and this greater understanding means the fireball spell gets an extra damage or range boost, then that would be synergy.
Going by one of the definitions of synergy ('combined action' rather than 'cooperative action') it could technically include combos (or hit and run tactics, etc), but at least to me if it is used in that way it just sounds like 'management speak' trying to make something sound more impressive than it is.