That Func was made in ~2012/2013 before the Hook for the UseSkill-Function was implemented.
However ... there have been 2 SendPackets for UseSkill:
Header 0x20 has been used to use a Skill on a Target.
Header 0x3F has been used to use a Skill on Non-Target/yourself.
The first one has been deleted with the 03/23/18 Update.
However, I recommend UseSkill() over SendPacket().