I think that you have completely misunderstood the article which comes to just the opposite conclusion: that contraception IS always gravely sinful!
I got this answer from Elaine76 who is the NFP consultant for the Catholic Mom Community. This her response to your post:
I think you may have misunderstood the article you link.
The author of the article asks the QUESTION, “In the Church’s universal Magisterium, is the sinfulness of contraception considered grave or light matter?”
And it does mention that some people claim that the Church no longer considers contraception a grave matter.
But then the author comes to the CONCLUSION that,
“It is evident then that in the doctrine taught by the Magisterium the use of contraceptives in performing the conjugal act constitutes *gravely sinful matter*, in addition to being an “intrinsically immoral” action, and thus is *never* permissible for any reason or purpose whatever.”