How can you have שעטנז in 100% linen?

 

How can you have שעטנז in 100% linen? In the סליחות למוסף יו"כ*, it states "כתנת ארוגה פשתים וצמר". The כתנת of the כהן גדול was שעטנז and serves as a כפרה for the sin of planting כלאים. However, an apparent question arises: the כתנת contained no wool, it was crafted entirely from linen. How then, could it be שעטנז? The חתם סופר cites Hachcham Wolf Heidenheim, explaining the פייטן with a Halacha in רמ"א. If an individual wears two pieces of clothing, one made of wool and the other of linen, and the lower garment cannot be removed without first taking off the upper one, then this constitutes שעטנז. The כהן גדול wore a מעיל תכלת (wool) over the כתנת (linen). This would be considered שעטנז. In most Ashkenaz communities, they have additional Selichos during the daytime. The כהן גדול was permitted to wear שעטנז