Protestants are Christians (and all men are our brothers) and sanctification through Bible alone is possible, — that is the “efficient through some respect” part. This does not mean that Protestantism is a valid way to heaven, — it is not. It is a sect based on lies about the Holy Gospel and self-interest of the pastors who ought to know better — they went to seminaries.
It seems to me that if your infallibly assured Magesterium can accept that those who are believers in Jesus Christ as Savior are indeed fellow Christians, then I wonder why "faithful" Roman Catholics have such a hard time with it. The category "Protestant" is only used to designate Christians not of the "Catholic" variety. No one has EVER said Protestantism is a "valid way to heaven" as it is ONLY through faith in the salvific work of Jesus Christ for our sins that ANYBODY can gain eternal life in Heaven. Roman Catholicism is also not a valid way to heaven for the same reason. That you believe non-Catholic denominations are "sects based on lies about the Holy Gospel" proves a biased view that must ignore the very truth of the Holy Gospel derived from the Holy Scriptures.