Well, it's implicit the quote when you consider everything else.
The Doctor says if there were other Time Lords, he would sense them.
He believed there were no more Time Lords because he could not sense them.
The Doctor received the message leaving him to believe there was a Time Lord in a pocket universe.
He thought this was a possibility even though he could not sense the Time Lord*
Therefore, he could not sense Time Lords in pocket universes.
* We know this both because he didn't think there were more previously and because there was no Time Lord to sense.
The scene doesn't make sense unless there's a pocket universe exception to the rule. It's primarily the reason I (among others) had hoped that Romana had fled the Time War to E-Space, which could be outside of his ability to sense as well.
If they address the Doctor sensing Time Lord things and explicitly adopt Christopher's idea specifically or my more general version of the same thing, would you believe that the Doctor can't sense Time Lords in other circumstances besides FOB watches?