It is worth noting this summary of the Tosafos [here][1] which adds an additional commentaries note demonstrating that others do read is like an 'Ⲭ'. It writes there:

> NOTE: [Rashi in Kerisus (5b DH k'Min)][2] and the Rambam ([Hilchos Klei ha'Mikdash 1:9][3]) drew the form of an "X". Tif'eres Yisrael (ibid.) concludes like the Rambam.

So it could well be that Tosafos was just not familiar with Greek language, whilst people like Rashi and Rambam were more knowledgeable in this regard.

  [1]: https://www.dafyomi.co.il/menachos/tosfos/mn-ts-075.htm
  [2]: https://www.sefaria.org/Keritot.5b.21?lang=bi&p2=Rashi_on_Keritot.5b.21.2&lang2=bi
  [3]: https://www.sefaria.org/Mishneh_Torah%2C_Vessels_of_the_Sanctuary_and_Those_who_Serve_Therein.1.9?lang=en&with=all&lang2=en