5 years continuous residence is typically the requirement to apply for permanent residence and ultimately citizenship.

It's more typically known as the "5-year route" and it means that after 5-years of living/working/residing in the UK they become eligible for permanent residence (which is known as "indefinite leave to remain"). So yes - it is a genuine and good offer. There will be catches though - the visa's will likely cost a fair bit of money (for example a UK spouse visa costs over £1000, and must be renewed when it is 2.5 years old) - I would imagine the BNO HK visa will be similar in cost. Similarly, the ILR (permanent residence) visa's cost around £2500. It is also possible that the BNO HK visa may have criteria on it such as a requirement to prove your current salary or available savings/funds are above a certain amount.

5 year Leave to Remain, after 2 years LTR they can apply for settled status and after one year of SS they can apply for citizenship.

That's not nothing.