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

NOTE: Rashi in Kerisus (5b DH k'Min) and the Rambam (Hilchos Klei ha'Mikdash 1:9) 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.

Although having said this, the summary in the same link does seem to interpret Tosafos' understanding in the same manner - i.e. that it did resemble the X formation:

Explanation #4 (Aruch): A Beraisa in Kerisus (5b) teaches that kings they anoint (put the oil in a shape) like a crown, and Kohanim Gedolim like a Greek Chai. I.e. [Moshe] poured oil on Aharon's head, and it descended in two directions, like two legs (coming out of the torso), and this is its form. (Musaf ha'Aruch and Tif'eres Yisrael (Zevachim 10:37) drew the form of an upside-down V.)

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

NOTE: Rashi in Kerisus (5b DH k'Min) and the Rambam (Hilchos Klei ha'Mikdash 1:9) 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.

Although having said this, the summary in the same link does seem to interpret Tosafos' understanding in the same manner - i.e. that it did resemble the X formation:

Explanation #4 (Aruch): A Beraisa in Kerisus (5b) teaches that kings they anoint (put the oil in a shape) like a crown, and Kohanim Gedolim like a Greek Chai. I.e. [Moshe] poured oil on Aharon's head, and it descended in two directions, like two legs (coming out of the torso), and this is its form.

It is worth noting this summary of the Tosafos here 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) and the Rambam (Hilchos Klei ha'Mikdash 1:9) 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.

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

NOTE: Rashi in Kerisus (5b DH k'Min) and the Rambam (Hilchos Klei ha'Mikdash 1:9) 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.

Although having said this, the summary in the same link does seem to interpret Tosafos' understanding in the same manner - i.e. that it did resemble the X formation:

Explanation #4 (Aruch): A Beraisa in Kerisus (5b) teaches that kings they anoint (put the oil in a shape) like a crown, and Kohanim Gedolim like a Greek Chai. I.e. [Moshe] poured oil on Aharon's head, and it descended in two directions, like two legs (coming out of the torso), and this is its form. (Musaf ha'Aruch and Tif'eres Yisrael (Zevachim 10:37) drew the form of an upside-down V.)