Baptism of Desire, even if such is visible (say, in the case of a martyr) is not an exception to EENS. One would have died with the grace of Baptism but not its character. Why is this an issue?
The Letter of the Holy Office makes it an issue. It considers the baptism of desire an exception to the literal interpretation of extra ecclesiam nulla salus according to Fr.Leonard Feeney.
Archbishop Lefebvre and the SSPX make it an issue when they conside the baptism of desire as explicit, visible and so an exception to extra ecclesiam nulla salus according to Fr.Leonard Feeney.