That apparently isn't true according to balashon.com:
"But we also find a slightly different form: קשט, with a tet instead of a tav. An example can be found in Yerushalmi Taanit 69b, and perhaps in Tehillim 60:6.
But we are also familiar with the root קשט having another meaning: truth. We find this meaning in Hebrew once in the Bible, in Mishlei 22:21."
The word קשט exists in Hebrew. https://www.pealim.com/search/?q=%D7%A7%D7%A9%D7%98
The word לקשט means to decorate. Also if you see קשט with a ט instead of a ת meaning an archer or a bow, keep in mind that sometimes Hebrew words are spelled differently (not so much in modern Hebrew) and it's not considered incorrect. I've encountered it with ט and ת and with ש and ס. Still it's better to stick to the most commonly accepted spellings today, and whatever you do, never, never argue with your child's Hebrew teacher!