Hard to say how common that sampling method is, but Reuters has a 2019 piece
Switzerland-based Education First (EF), which runs 300 schools across 50 Chinese cities, has seen a “significant” increase in detentions in China for alleged offences including drugs, fighting and cybersecurity violations, according to a June 27 internal notice sent to employees and seen by Reuters.
It said EF staff had been “picked up by police at their home and work as well as in bars and nightclubs and have been questioned and brought in for drug testing”. The notice said the school had also received warnings from embassies about the rise in arrests.
[...]
Three former teachers from two schools in Beijing and Shanghai who were detained for between 10 and 30 days before being deported this year say authorities drug-tested teachers multiple times within weeks of arrival and conducted extensive interrogations.
One of the three, a 25-year-old Florida man who was deported in May after a 10-day detention in a Beijing jail, said he and a colleague underwent a urine screening on their first day in China, which came back clean, but were detained after a surprise workplace test two weeks later showed traces of cannabis in his hair.
[...]
The behavior of foreign teachers in China was thrust into the spotlight last month when 19 foreign citizens, including seven who worked for EF, were arrested in the eastern city of Xuzhou on drug charges.
The first half of that is actually pretty similar to what the HB blog says on another page
Chinese police going to nightclubs and rounding up foreigners and then cutting their hair for drug testing.
So on one hand it seems to involve presence in night clubs rather than getting samples at the workplace... but the latter half of the Reuters story does mention a hair sample being collected at the workplace. Of course it's hard to be sure how general the practice is. As the Reuters story noted, Chinese authorities tend to ramp up their scrutiny of foreigners on certain occasions, like the Chinese economy slowing down e.g. due to a trade war. The final part of that article mentions that all these enhanced drug screenings took place in Xuzhou, so it's unclear how common they'd be in other parts of China, even in the same time frame.
For a bit more context, a 2019 FP article notes that the crackdowns on foreigners for cannabis have apparently intensified since its legalization
aborad:
In June, deputy director of narcotics control Liu Yuejin publicly denounced marijuana legalization as a “new threat to China,” claiming domestic usage had grown by over 25 percent in 2018. But the scale of the marijuana menace—an estimated 24,000 users, according to Liu, out of a population of 1.4 billion—reflects Beijing’s wobbly logic, in which Chinese society is besieged with threats from afar that the government has entirely under control. China doesn’t have a drug problem, the narrative pleads; foreigners do.
[...]
Last year, Beijing’s food and beverage scene was hit with a wave of raids on foreign owners and bar staff. Those who survived the cull described officers arriving at homes in the middle of the night, demanding urine tests, then detaining and expelling anybody who failed the tests from the country without further notice. Popular businesses, including a smoothie bar and cafe, were left ownerless overnight, but the dramatic deportations weren’t publicized for fear of further attention in the expat community.
That’s not a luxury afforded to people like Lance, a Nigerian student yanked out of class and thrown into an interrogation room by Beijing police, after failing a urine test in March. African staff and students at Chinese schools are tested weekly for drugs, a Ghanaian kindergarten teacher told me: “Because we’re black,” she shrugged. “The white guys get tested every couple of months.” Lance (a pseudonym) said he was subjected to hours of hostile interrogation.
So the crackdown doesn't seem restricted to teachers but also foreign students, workers in bars, etc., although hair sample testing seemed uncommon until 2019.
