The wording is "could lead to a doubling (or more) of the warming rate" which suggests a one-time bump to the slope and a significant long-term impact on global heating.
The doubling of the rate of warming only applies for the first year or two. Based on this paper, over the period 2020-2030, the impact of ship fuel regulations is warming of about 0.12K. The long term temperature trend is around +0.19K over this 10 year period.