It does not mean that exactly.
It means that if you are currently flying the 777 for a country with a licence approved by ICAO (HK is) with a rating on it, then you can covert that rating and add the privileges to the EASA Part FCL licence once you have jumped through the hopes as specified in the CAP 804 section 4 Part Q Subpart 2 page 2.
On the pdf document it's on page 576.
I just did a chap's LST on the 787 from the ME and he had to go through an ATO here in the UK. They accepted all of his training, the wanted evidence of his time on type (log book and a letter from his employer) and evidence of a current LPC on Type (his Qatari licence).
The Skill Test was 3 hours. He then went down to the ministry and got his rating and Part FCL licence issued on the same day.
What has been very helpful in the last few years is the instrument rating validity extension. Some pilots had left their UK licence lapse for more than 7 years. The CAA wanted them to re sit the IR part of their written exams.
They now accept IACO approved licences as equivalent thus no need to re-sit exams if in current flying practice.
Best of Luck
I am not 777 qualified (737 and 787)) but Boeing and BA in the UK can help you with the 777 rating conversion.