Not all RCIA programs/people are created equal. I would not assume that just because some one is on an RCIA team that they know any more than anyone else. Likewise not all Priests know everything one would expect them to know.
I do not know of any prohibition in Canon Law that forbids a Christian from availing themselves of the Sacrament of Penance. Canon Law 987-991 deals with the Penitent and uses the term “a member of the Christian faithful”. Canon 204 defines the term “Christian faithful”. Canon 205 and 206 address Catachumens specifically. So in this case I would have to side with the Priest. Since I am not a Canon Lawyer and have not studied this in-depth I may be wrong, so take my opnion with a grain of salt.
Like I said I haven’t seen anything in writing, that says those Baptized can’t have a valid confession until the are in full communion, and if you are not Baptized then once you are all previous sins are washed away…
Canon Law Link
vatican.va/archive/ENG1104/_INDEX.HTM
As for RCIA team members, some members are more knowledgable then others, they are just members of the body that are walking with you on your journey into the Church. The conversion experience is just as much theirs as it is yours and vice versa. It is a time for all involved to grow in faith and knowledge of our faith.
“Christian Faithful”.
Can. 205 Those baptized are fully in the communion of the Catholic Church on this earth who are joined with Christ in its visible structure by the bonds of the profession of faith, the sacraments, and ecclesiastical governance.
Can. 206 §1. Catechumens, that is, those who ask by explicit choice under the influence of the Holy Spirit to be incorporated into the Church, are joined to it in a special way. By this same desire, just as by the life of faith, hope, and charity which they lead, they are united with the Church which already cherishes them as its own.
§2. The Church has a special care for catechumens; while it invites them to lead a life of the gospel and introduces them to the celebration of sacred rites, it already grants them various prerogatives which are proper to Christians.
THE MINISTER OF THE SACRAMENT OF PENANCE
Can. 980 If the confessor has no doubt about the disposition of the penitent, and the penitent seeks absolution, absolution is to be neither refused nor deferred.
THE PENITENT
Can. 987 To receive the salvific remedy of the sacrament of penance, a member of the Christian faithful must be disposed in such a way that, rejecting sins committed and having a purpose of amendment, the person is turned back to God.
Can. 988 §1. A member of the Christian faithful is obliged to confess in kind and number all grave sins committed after baptism and not yet remitted directly through the keys of the Church nor acknowledged in individual confession, of which the person has knowledge after diligent examination of conscience.
§2. It is recommended to the Christian faithful that they also confess venial sins.
Can. 989 After having reached the age of discretion, each member of the faithful is ob-liged to confess faithfully his or her grave sins at least once a year.
Can. 991 Every member of the Christian faithful is free to confess sins to a legitimately approved confessor of his or her choice, even to one of another rite.