Technically only some of HK was under the lease, some was indefinitely controlled by the British. However, you're still right because of the military force difference.

Lord hear me now
Junk boats and English boys
Crashing out in super marts
Electric fences and guns

You swallow me
I'm a pill on your tongue

Here on the nineteenth floor
The neon lights make me come

And late in a star's life
It begins to explode
And all the people in a dream
Wait for the machine

Pick the shit up leave it clean

Kid, hang over here
What you learning in school?
Is the rise of an Eastern sun
Gonna be good for everyone?

The radio station disappears
Music turning to thin air
The DJ was the last to leave
She had well conditioned hair,
Was beautiful, but nothing really was there

TL;DR: Only New Territories is leased for 99 years, remaining part of Hong Kong is ceased. So the statement is partly correct.

About "China only leased Hong Kong to Britain" is partly correct, and here is why:

After the first opium war, Hong Kong Island (including Aberdeen Island/Ap Lei Chau and surrounding islands) was cesed to British under Treaty of Nanking, which stated that

His Majesty the Emperor of China cedes to Her Majesty the Queen of Great Britain, &c., the Island of Hong-Kong, to be possessed in perpetuity by Her Britannic Majesty, Her Heirs and Successors, and to be governed by such Laws and Regulations as Her Majesty the Queen of Great Britain, &c., shall see fit to direct.

Wikipedia source

Treaty source

Then, after the second opium war, Kowloon Peninsula was cesed under Convention of Peking, which stated:

With a view to the maintenance of law and order in and about the harbour of Hongkong, His Imperial Majesty the Emperor of China agrees to cede to Her Majesty the Queen of Great Britain and Ireland, and to Her Heirs and Successors, to have and to hold as a dependency of Her Britannic Majesty’s Colony of Hongkong, that portion of the township of Cowloon, in the province of Kwangtung, of which a lease was granted in perpetuity to Harry Smith Parkes, Esquire, Companion of the Bath, a Member of the Allied Commission at Canton, on behalf of Her Britannic Majesty’s Government, by Lau Tsung Kwang, Governor General of the Two Kwang.

Convention source

Wikipedia source

Note: Kowloon Peninsula was leased initially, but latter ceased to British, see cite 1.

So , the remaining part of Hong Kong, the New Territories, is leased under The Convention Between Great Britain and China Respecting an Extension of Hong Kong Territory , but English translation is not available, so here is a machine translated version:

After many years of investigation, it was learned that a non-extension site in Hong Kong was not sufficient to defend, and now the Chinese and British governments have agreed on a general plan to expand the British boundary as a new leased land according to the attached map. The detailed demarcation shall be drawn after the two countries have sent personnel to survey and demarcate it, with a deadline of 99 years....

Convention source

Wikipedia source

To sum up, only the New Territories is leased and the remaining part is ceased. For more detail, please read Cession of Hong Kong from wikipedia (Chinese Version only, you may use machine translation)

Map of Hong Kong

Cite 1:

The Arrow Incident in 1856 triggered the Second Opium War. On 18 March 1860, the British 44th Regiment occupied Tsim Sha Tsui, and on 20 March the Governor of Liangguang, Lo Chong-guang, agreed to "temporarily lease" Kowloon.

Chapter 2 British Occupation of Hong Kong and the Establishment of the Colonial System from A Brief History of Hong Kong—From Ancient Times to the 1997 Handover, by 劉智鵬; 劉蜀永, ISBN 978-962-937-420-4.

To pretend that the Brits "leased" Hong Kong like it's a mutual transaction and not because of colonization and the Opium wars is utterly idiotic.

The Brits had a choice when they funneled Opium into China in exchange of silver. They had a choice when they pointed a gun to the Chinese for taking the New Territories.

The Brits had a choice... is it a moral choice? No... but they had a choice.

The Brits taking Hong Kong from China is a huge stain in history. But to pretend that Brits are going to have any say after giving the territory back - also sounds ridiculously stupid.