So they had originally fucked them over as soon as the USA said jump, no?

No.

It's important to keep in mind that China is still vulnerable to economic sanctions and other forms of US aggression, as well. Most countries, even those under US aggression, do not have full latitude to defy US sanctions, which is why they work. Venezuela's still in a rough state and is even more heavily sanctioned than they were last year, and China's willingness to defy these sanctions now that they're in a position to do so, which only a handful of other countries have been willing to do, provides an essential lifeline.

Another example would be China's support for Venezuela in dealing with COVID when the sanctions have prevented them from getting medical supplies through other means.

http://spanish.xinhuanet.com/2020-05/13/c_139052631.htm

In the Venezuelan case, the Minister for Foreign Relations pointed out that from the first day, the Asian country "made itself available, an airlift was established and this is the fifth flight."

He detailed that in its entirety, China has sent more than one million rapid tests, 150 thousand molecular test kits, more than eight million masks, almost two million disposable gloves, more than 135 thousand protective suits, more than 23 thousand infrared thermometers and 14 thousand protective glasses.

The Minister for Health, Carlos Alvarado, stressed that the more than 46 tons of medical supplies will be distributed in all health centers in the country.

"They are going to strengthen our diagnostic capacity and our power to protect ourselves against this terrible pandemic," he said.

He specified that supplies arrived for the performance of 80 thousand diagnostic tests and their reagents. "The only way to fight and defeat the coronavirus is with the union of all the countries of the world. We are very happy with this shipment that we are receiving," he said.

http://vicepresidencia.gob.ve/embajador-li-barong-venezuela-y-china-son-un-ejemplo-a-seguir-ante-la-comunidad-internacional/

This Saturday the sixth plane from the People's Republic of China arrived in Venezuelan territory, loaded with medical supplies, intended for the prevention and control of Covid-19, this as part of the air bridge established between both nations.

This is in addition to diplomatic support, arms sales, and loans - which they absolutely need and which do not come on terms of structural adjustment - that they've been giving Venezuela since Chavez came to office.

Similar situation with Cuba.

A total of 286 Chinese donors raised more than 140,000 yuan (US$19,823), purchased 420 pieces of medical protective outfits and 38,750 masks and sent the medical supplies to Cuba successfully with the help of the Cuban embassy in China, Chen Ke, who initiated the donation campaign, told the Global Times. ... Cuba Si, a Cuban national media outlet, reported the Chinese people’s donation campaign, which moved many Cubans. Cuban teachers and friends of the Chinese donors contacted them after reading the news and appreciated their love and assistance, the report said.

Wikipedia had this funny line:

In the late 1990s, China provided the Cuban government with equipment to block signals from Radio Martí.

Similar situation in Bolivia with regards to development aid and loans.

Morales praised China's relations with Latin America, and said that “China's support and aid to Bolivia's economic and social development never attach any political conditions.”

China also has a good relationship with the government of Nepal, and the one Nepali communist I've talked to said that China's seen as an important buttress against Indian aggression over there, but I don't really know the details.

And then there's the fact that the DPRK basically wouldn't exist anymore if not for China's support.

Anyway, this is just a few of the things I remembered off the top of my head. They could probably do more, and I wish they did, but there's no question that every other socialist nation would be in a far, far worse position without China's support.