Currently it is winter in Antarctica. According to news I read, Antarctica has set a new record high temperature, above 18 °C. How did this temperature records occur?
You just read the news too fast. WMO announced that, after evaluation by a committee, they have validated the 18.3°C temperature recorded in February (i.e., in summer) last year:
GENEVA, 1 July 2021 (WMO) - The World Meteorological Organization (WMO) has recognized a new record high temperature for the Antarctic continent of 18.3° Celsius on 6 February 2020 at the Esperanza station (Argentina).
Supplemental to Jean-Marie Prival's answer:
The February 2020 event was already discussed here in a question posted the same day this went public: What is the second thermometer in the image from the Esperanza Antarctic temperature record?
The tweet contained an image of a literal analogue thermometer which did show the 18.3°C... but curiously enough, also another thermometer showing merely 10.0°C.
The answer to that question is that this pair forms a wet-dry bulb system for measuring humidity.