I believe the card content answers only a question referring to the specific situation where a patient presents at exactly 2 hours post-ingestion of exactly 20 tablets of acetaminophen.
Suggested information is about recommendation after 4 hours. After learning the content in the card (administering activated charcoal, then measuring serum acetaminophen levels at 4 hours), the question may arise what to do if the patient presents after 4 hours. The added information seeks to answer this question, in addition to situations of different dosages acetaminophen.
This is relevant, as the same way a Step question could be the content of the card, it could also be "What is the next step....ingesting 30 tablets 6 hours ago..." where the card is no longer relevant.
The suggested change addresses such alternative questions as well, without the need for additional anki cards.
Source: Society Guidelines - https://1c64b919-581c-43a8-bb17-009825d892ca.filesusr.com/ugd/363318_acfc982c55b84f86a85753800ccf3f18.pdf