In The Half-Blood Prince, when Harry used the spell Sectumsempra on Malfoy, was Snape able to reverse the effects of the curse because he was the one who created it?
Snape reversed it using the counter-curse.
Snape reversed the effects of the Sectumsempra spell Harry used on Draco using a counter-curse. His success doesn’t seem to be related to his being the inventor of Sectumsempra - presumably anyone who knew the right counter-curse to cast and was sufficiently skilled at casting it could also reverse its effects.
“The door banged open behind Harry and he looked up, terrified: Snape had burst into the room, his face livid. Pushing Harry roughly aside, he knelt over Malfoy, drew his wand and traced it over the deep wounds Harry’s curse had made, muttering an incantation that sounded almost like song. The flow of blood seemed to ease; Snape wiped the residue from Malfoy’s face and repeated his spell. Now the wounds seemed to be knitting.
Harry was still watching, horrified by what he had done, barely aware that he too was soaked in blood and water. Moaning Myrtle was still sobbing and wailing overhead. When Snape had performed his counter-curse for the third time, he half-lifted Malfoy into a standing position.”
- Harry Potter and the Half-Blood Prince, Chapter 24 (Sectumsempra)
However, Snape likely was one of the few wizards who would recognize the effects of Sectumsempra, since he would be familiar with the effects of his invention and it was otherwise not a widely known spell. Therefore, another wizard might not have recognized its effects or known what counter-curse would reverse it.