This is an explanation of antidoron that I’ve never heard before.
I’ve always learned that antidoron was traditionally given to those who could not receive the Eucharist for some reason, whether they were not properly disposed or some other reason. It is true that catechumens had traditionally been dismissed during the point at which antidoron was distributed, so it would not have been available to non-Catholics (or Orthodox). I’ve also been told that it was frequently taken home to be given to the sick, or to be consumed throughout the week.
This excerpt from the Orthodox Wiki site seems to confirm my understanding:
Antidoron (from Greek, meaning “instead of the gifts”; in Arabic,
qurban) is the remaining bread from a loaf of
prosphora after the Lamb has been removed for the Holy
Eucharist. In Byzantine practice, it is blessed during the
megalynarion to the
Theotokos immediately after the
epiclesis in the
Divine Liturgy and is given by the
priest to the faithful after the service. Historically, it was distributed only to those who had not received ‘the Gifts’,
Holy Communion so that they would receive a Blessing in place of
Holy Communion but this practice has changed over time and all those present at the
Divine Liturgy receive
Antidoron as a blessing.
Of course, many traditions have evolved in different times and places and different explanations have been given. Your explanation could be equally correct, but given the meaning of the word, mine makes more sense to me.