Mr Rees-Mogg is expressing what would be the popularly viewed position of the RCC.
Are they viewed "in the same light"? Morally, they are both viewed as "gravely disordered".
However, they are two distinct things.
There is a small semantic difference in the way "conception" is used in the medical world and in the Church. In the modern medical view, conception occurs when the fertilized egg implants itself in the endometrium. The more traditional, and Catholic view, is that conception occurs at fertilization.
Therefore, from a medical point of view, anything which prevents fertilization or implantation is deemed "contraceptive". Pregnancy only occurs when implantation occurs.
From the Catholic perspective, oral contraceptives - including the "Plan B" morning after pill - are potential abortifacients. I say potential because, if they fail to prevent fertilization, they prevent implantation of a fertilized egg.
WebMD note on how the contraceptive Apri works (this is true of all hormonal contraceptives):
Apri is a combination birth control pill containing female hormones
that prevent ovulation (the release of an egg from an ovary). Apri
also causes changes in your cervical mucus and uterine lining, making
it harder for sperm to reach the uterus and harder for a fertilized
egg to attach to the uterus.
AMA document discussing the Plan B pill:
The Plan B pill is a post-coital contraception method which
transiently provides a high dose of (1) combined estrogen and
progestin or (2) progestin-only to inhibit or delay ovulation--or
induce minor changes to the endometrium to inhibit ovum implantation; therefore, it cannot terminate an established
St. Jerome (Letter 22) seems to consider them to be equal in gravity:
But others drink potions to ensure sterility and are guilty of
murdering a human being not yet conceived. Some, when they learn they
are with child through sin, practice abortion by the use of drugs.
Whereas St. John Chrysostom seems to consider contraception more grave than abortion:
Why do you sow where the field is eager to destroy the fruit? Where
there are medicines of sterility? Where there is murder before birth?
... Indeed, it is something worse than murder and I do not know what
to call it; for she does not kill what is formed but prevents its
formation. What then? Do you condemn the gift of God, and fight with
Chrysostom's point is that contraception is worse than abortion because, while abortion is the destruction of life, contraception acts to deny the possibility of being.