Does this not mean that hand holding is a liturgical gesture which is not in the rubrics? And that it was not introduced officially?
I would assert that hand-holding, per se, isn’t a liturgical gesture; after all, then
all hand-holding inside church during Mass would be “liturgical gesture”, right? And that’s just a ludicrous suggestion!
Therefore, what they’re saying is that hand-holding,
at that point in the liturgy, is being introduced specifically
as a liturgical gesture, and with the goal of
replacing the sign of peace. It’s this combination – the intent of inserting a liturgical gesture, and with the goal of deleting another, that makes it something that the Church is reprobating. (Not general hand-holding as such. So, don’t worry – you can still hold your spouse’s hand during the Liturgy.

)
Generally speaking is it OK to make personal and spontaneous additions to the liturgy on your own initiative without any official permission?
It depends on what you mean. There are spontaneous acclamations and gestures made all the time. The question becomes ‘intent.’
And if you do so, would it be considered a liturgical abuse?
Not a priori, I’d assert.
I don’t think the Church cares if a couple spontaneously decides to hold hands in the Our Father. But I was in one parish a few decades ago where it was practically mandatory.
Notice that the Church’s issue isn’t with hand-holding as such. It’s the fact that it had been attempted to be introduced as a different gesture than one already present in the liturgy, with the intent to eliminate the other gesture.
That’s what the Church is responding to, here!
However, the orans is a definitive priestly posture. How else do you interpret that line?
By understanding it in its context. Does the raising of one’s hands during the Lord’s Prayer represent an intent to “use gestures which are proper to the priest celebrant”? Is the intent to “quasi-preside at Mass while leaving only that minimal participation to the priest which is necessary to secure validity”?
If not – and I’d argue that this is
not the case here (most clearly with respect to the latter, and perhaps even the former!) – then I’d assert you’re interpreting this line far too aggressively.