GMT+8, the name (Mandarin/mixed-dialect first name + Hokkien surname) and most importantly the circumstance that the user was seen connecting from a likely VPN exit or hosted server in Singapore [1] are all consistent with a Singaporean identity, whether real or cover. For that reason, the opinion of [1]'s source that the name sounds fake should also be taken with some amount of salt (Mainland Chinese are not always well-informed about Diaspora Chinese communities). Worth noting that many of the holidays mentioned, likewise, are not public holidays in SG [2] - Jan 24th was, but Jan 22nd (listed as working) was a Sunday, inconsistent with a "working on regular working days" pattern most anywhere.
Would be interesting to see a greater corpus of Jia Tan's writings, especially older ones. There are fairly distinctive idiosyncrasies that may be used to distinguish Singaporeans, Mainland Chinese and speakers of various Slavic languages (and Anglo natives).
[1] https://boehs.org/node/everything-i-know-about-the-xz-backdo...
[2] https://www.mom.gov.sg/employment-practices/public-holidays