On one side we have George Russell citing Lateran IV:
Nobody can effect this sacrament except a priest who has been properly ordained according to the church’s keys, which Jesus Christ himself gave to the apostles and their successors. [....] To me this is fairly clear. Whatever becomes of bread consecrated by Anglican priests (who do not simultaneously happen to be Catholic priests), it is not the Body and Blood of the Lord, and Anglican Eucharists are not a sacrament.
On the other side, Al Kimel:
George, the passages from Lateran IV and the CCC simply identifies one of the necessary conditions for a valid Eucharist. In the absence of the fulfillment of this condition, the Catholic Church cannot and does not affirm that a sacrament occurs. I agree. So does Fathers Coventry and Hughes. But this is not identical to denying the possibility that God may, in his gracious freedom, answer the prayers of, say, a Lutheran congregation and bless them with the gift of Christ’s Body and Blood. To deny this possibility is to deny God’s freedom. [....] I am responding here strongly because it is very important not to misrepresent Catholic teaching on this point. Many readers of Pontifications are seriously examining the claims and teachings of Catholicism. Misrepresentations of authoritative Catholic teaching undermine the evangelistic purpose of this site.
We're still having a problem here with this word "valid". Under Al's program, one can replace "X is not valid" with "the Church does not acknowledge X." That tempts me to shrug and reply, "too bad for the Roman church," but there are two other points here.
First, there is Al's comment about the importance of conveying the correct Roman teaching. Now, looking at the Lat. IV statement, I don't read it as being hedged in the way that Al interprets it. Indeed, the paragraph in which the quoted statement appears begins with a strong, Cyprianic statement denying salvation outside the church. I believe the council's claims to be wrong for exactly the reasons that Al gives, but that isn't the point. I believe that Russell correctly relates what the council taught, and I do not accept the implication from Al that the council intended to teach otherwise.
The impression I get instead is that eventually the church had to back down from the rigorous Cyprianic insistence of sole proprietorship. But the consequence of this is having to abandon the use of the word "invalid", at least in any sense stronger than "we don't acknowledge...". In any case the temptation to take the stricter interpretations of old church statements is clearly very strong, to the point where an outsider has to wonder who to trust.