I know the question is asking why Tosefot don't know what a Chi is. But I would like to expand your question and argue that not only to Tosefot not know what a Chi is, but they don't know what a Paleo-Hebrew Tau 𐤕 ( ת in Ashurit) looks like.
As others have mentioned the anointing of the bread is similar to the anointing of the priests brought down by the Rambam.
Klei Hamikdash - Chapter 1:9
How was the High Priest anointed? The oil should be poured on his head
and applied between his eyes in the form of the Greek letter chi,30
like this C as [Leviticus 8:12] states: "And he poured the anointing
oil on Aaron's head and anointed him to sanctify him." The kings of
the Davidic dynasty are anointed [with the oil] spread as a crown on
their foreheads.31 They should not be anointed on other places [on
their bodies], nor should one use an excessive amount of oil.
כֵּיצַד מוֹשְׁחִין אֶת הַכֹּהֵן. צָק אֶת הַשֶּׁמֶן עַל רֹאשׁוֹ
וְסָךְ מִמֶּנּוּ עַל גַּבֵּי עֵינָיו כְּמִין כִּי יְוָנִית כָּזֶה X
שֶׁנֶּאֱמַר (ויקרא ח יב) "וַיִּצֹק מִשֶּׁמֶן הַמִּשְׁחָה עַל רֹאשׁ
אַהֲרֹן וַיִּמְשַׁח אֹתוֹ לְקַדְּשׁוֹ". וּמַלְכֵי בֵּית דָּוִד
מוֹשְׁחִין אוֹתָן כְּמִין נֵזֶר עַל רֹאשׁוֹ. וְלֹא יִמְשַׁח
בִּמְקוֹמוֹת אֲחֵרוֹת וְלֹא יַרְבֶּה בְּשֶׁמֶן:
All of these quotes seem to have a precedent in Tanakh in which Ezekiel has a vision in which people to be spared are marked with a Tau. Since Ezekiel is one of the more priestly prophets it makes sense to use a symbol that the priests would be familiar with and that would have some religious meaning.
וַיֹּ֤אמֶר יְהֹוָה֙ אֵלָ֔ו עֲבֹר֙ בְּת֣וֹךְ הָעִ֔יר בְּת֖וֹךְ
יְרוּשָׁלָ֑͏ִם וְהִתְוִ֨יתָ תָּ֜ו עַל־מִצְח֣וֹת הָאֲנָשִׁ֗ים
הַנֶּֽאֱנָחִים֙ וְהַנֶּ֣אֱנָקִ֔ים עַ֚ל כׇּל־הַתּ֣וֹעֵב֔וֹת
הַֽנַּעֲשׂ֖וֹת בְּתוֹכָֽהּ׃ and the LORD said to him, “Pass through
the city, through Jerusalem, and put a mark on the foreheads of the
men who moan and groan because of all the abominations that are
committed in it.”
However, marking someone with a Tau doesn't make much sense because we don't see the shape of ת being used anywhere else. And clearly the mark is supposed to have some religious significance. But my argument is that the mark of Tau is actually a Paleo Hebrew Tau that is shaped like an X
This same X symbol is used to anoint new priests, and the wafer offering. So I believe that once Paleo Hebrew fell out of use the Rabbis started substituting the Greek Chi to describe the X symbol, not realizing we used to have a letter in our Aleph Bet that already had that symbol.
So to answer your question: Knowledge gets lost, and it's rarely restored. Even when we discover how things used to be we often ignore it and do things the way it makes sense to us now. With that logic in mind it doesn't come as a surprise that when the Gemara wants to describe what the X shape looks like they use a Greek example since most Jews at that time would not know that a Paleo Hebrew Tau (𐤕) looks like an X. Move forward some centuries and Tosefot's convoluted listing of possible examples make even more sense.