I agree the line between fantasy and sci Fi is very thin. In general I would say if the author intends for it to be a natural explanation like wormholes or something it's sci fi, if it's a magic teleportation spell it is fantasy. There is a series I read once Starships Mage which plays with this, it's hard sci fi for most things but magic exists and FTL is solved through teleportation. Everything non magic has to obey relativity and such.