A gifted character cannot just out xp into any supernatural ability they want, so there would be no precedent for allowing any character to do that.
I suspect the comment is to clear up the fact that the character with failed apprentice can take some supernatural virtues despite the fact that his gift has been damaged otherwise it would be possible for a player to beleive a failed apprentice could not take any supernatural virtues
We do not interpret it that way. Instead, if you choose virtues granting you Supernatural Abilities, their source might be explained in this way.
If I recall correctly (but I'm relying fully on memory and it's been several years) the wording of this passage has been maintained from previous editions of the game, when characters with the Gift could not simply learn new Supernatural Abilities from a teacher as they can in 5th edition.
Actually, there is more than precedent if this is a starting character:
So, it is a reasonable question. I would not want to see it as a cheap way for a companion to pick up a major virtue, though. But if the group is OK with picking up a single ability, that wouldn't be an unreasonable interpretation of the virtue.
Yes, if the troupe allows that, a wonderful combination is Failed Apprentice and Apt Student. That's a cost of two minor virtues. Now you can begin with one supernatural ability. Leave it at rank 1. Then start finding others to teach you. The quality would start at 13 (3+6+5-1) and drop by 1 for each supernatural ability you pick up; even a teacher with Communication -5 can get you somewhere in the beginning. Hopefully the storyguide would make it hard for a player to do so if the troupe allows this.
It is possible for magi to do this, but they start with at least -15 instead of -1, so it's not nearly as easy. Also, they don't benefit from those supernatural abilities so much as non-magi do. The magi are probably better off putting the experience into their Arts.