It's not a paradox because we see the start and end and the changed timeline where the Doctor does not step on the thing on the ground because information is introduced.
Putting aside the strong hints that ruby herself is likely a paradox —
Ruby is told of events further down the timeline. Ruby subverts those events, meaning she can never be told of them in order to prevent them. When Ruby subverts those events, she is inexplicably given the Weeping Angel treatment, in order to come back and prevent the causal event (the Doctor breaking the Faerie Ring — which that is a very unusual example of, since it’s usually not man made) which means the events that were subverted now must play out. Except, the causal event for Ruby’s loop is also the causal event — mad Jack being released — for those events to play out. In order for events to play out, in order for future Ruby to subvert them, the causal event must not be allowed to occur in the first place. Except that it has, in order for Ruby to know to subvert it. All of which is tied to the existence of older, time-displaced Ruby, who comes from a timeline she herself aborts, which in turn means the timeline that she subverted comes back into existence — which means she doesn’t exist. Which means she can’t stop it existing, or being subverted.
It’s a real cats cradle job, because it isn’t a bootstrap paradox, because there are three causal events interacting, and a fourth that we know less about — where the Doctor went. (Back into the Tardis to bust it up into a Paradox engine?)
And going back to those hints, there’s another big old ‘Ruby is her own mum’ hint here, which of course is very much a Time Travel Story legend. Douglas Adams has a take on that of course, and Red Dwarf did it if not first, then prior.
Which makes her an embodied bootstrap paradox, also at the nexus of this nexus of paradoxes.
There is no start and end of the changed timeline, because the closing of the loop requires that it be opened in the first place, which then closes it etc. It’s not like Blink, or Pandorica, or The One With Jesus Doctor And Captain Scarlet — there’s no rewritten timeline, because of the multiple and interlinked causal events across at least two timelines. Even if the fuzzy look on Old Ruby is BTTF style effect of her timeline being overwritten, her timeline still has to exist — otherwise events *before* what we see as the instigation of events can actually occur. I.e Mad Jack being released.
Old Ruby subverting the loop by preventing the nukes isn’t what changes the timeline, it’s her preventing the circle being broken. Except that still has to happen, because otherwise The Doctor can’t give it with ‘Spoilers!’
See?
Overlapping paradoxes, cancelling out their own resolutions.
