Your use of the word 'retiring player' seems to suggest you don't know the difference
The phrase retiring player as used in the Laws refers to am offside player getting himself back onside
For me,
"a retiring player" is, always has been and always will be, a player who is heading back towards his own goal line. A retiring player, may or may not be offside, or may or may not be simply getting himself into a support position or a defensive position. Just because he is retiring toward his own goal-line does not mean he is or was offside.
The OP used the term as I do. He called Blue 6 a retiring player...
("the ball is caught by Blue 6 who is retiring back")
Now in the OP's scenario, if Blue 6 had been offside
then that is what you would PK him for!!
ETA:
Here is a Law reference that uses the world retire, and which has nothing to do with offside...
[LAWS]9.B.3 THE OPPOSING TEAM
(a) All players of the opposing team must
retire to their goal line and must not overstep that line
until the kicker begins the approach to kick or starts to kick. When the kicker does this, they
may charge or jump to prevent a goal but must not be physically supported by other players
in these actions.[/LAWS]
In this case, retire means "go back". So, retire is not always used to denote an offside player going back
ETAA: But wait, there's more...
[LAWS]21.4 (g) Kick taken in the in-goal.
If a player retires into in-goal to take a penalty or free kick
awarded in the field of play and a defending player by foul play prevents an opponent from
scoring a try a penalty try is awarded
21.4 (h) Out of play in the in-goal.
If a player retires into in-goal to take a penalty or free kick
awarded in the field of play and following the kick the ball goes into touch-in-goal, or on or
over the dead ball line, or a defending player makes the ball dead before it has crossed the
goal line, a 5-metre scrum is awarded. The attacking team throws in the ball.
21.8 (g) Free kick taken in the in-goal. If a free kick has been awarded
and the player retires into
in-goal to take it and the opponents charge and prevent the kick from being taken, a 5-
metre scrum is ordered. The attacking team throws in the ball. If a free kick is taken in the
in-goal, an opponent who legitimately plays it there can score a try.[/LAWS]