The law says where players should be. It does not say that they have to be there before the ball can be thrown in. For example it says where a receiver should stand, but we all know you do not have to have one.
The Law is clear that a receiver is optional:
[LAWS]
If a team uses a receiver, then that player, must be positioned at least 2m back from team mates in the lineout, and between the 5m and 15m lines, until the lineout begins.[/LAWS]
Equally, it makes clear that a player at 3 + 2 is compulsory:
[LAWS] The team not throwing in
must have a player standing between the touchline and the 5-metre line on that team’s side of the line of touch when the lineout is formed. That player must stand 2 metres from the line of touch and 2 metres from the 5-metre line.[/LAWS]
There is a gap in the law. How far we should close that gap by interpretation is unclear (largely because the opposition usually makes a point of being there first).
A combination of the definition of a lineout:
[LAWS]The purpose of the lineout is to restart play, quickly, safely and fairly, after the ball has gone into touch,
with a throw-in between two lines of players.[/LAWS]
and Law 19.8(g):
[LAWS]A team must not voluntarily fail to form a lineout.[/LAWS]
make clear that the norm is that the ball is thrown in only when the lineout has been formed.