You need to read a little bit further:
Permission to enter requirements
If you have permission to enter or remain in the UK (for example if you have a UK visa) you do not need further permission to enter the UK from Ireland.
You only need to get permission to enter the UK when arriving from Ireland if:
[...]
you’re a visa national who doesn’t have a valid UK visa, or a visa granted under the British-Irish Visa Scheme (BIVS)
[...]
In these situations, you must either apply for:
- a visa before you travel
- permission to enter from a Border Force (immigration) Officer at the UK border
The second option (permission to enter from the immigration officer) is not practical since
You may not be able to get permission to enter the UK from a Border Force Officer if you enter the UK from the CTA. This is because there are usually no immigration controls on these journeys, and none on the land border between Ireland and Northern Ireland.
and is in any case reserved for exceptional situations where a visa national is granted permission to enter without an entry clearance (visa).
If you are a visa national and do not have a UK visa (or another document, such as a BIVS visitor's visa, entitling you to deemed leave to enter the UK), it is against the UK Immigration Rules for you to enter Northern Ireland from the Republic.
https://www.gov.uk/guidance/travelling-between-the-uk-and-ireland-isle-of-man-guernsey-or-jersey#travelling-from-ireland-to-the-uk