[3] The use of the abbreviation is to prevent the phrase "to come" from being mistaken as a deliberate part of the text.
Since very few English words feature the letter combination TK, it is more easily searchable than TC.
The abbreviation TK and the repeated TKTK are "unique and visually arresting" strings that are both easily seen in running text.
[1] This shorthand is described as "imprecise" in a Q&A on the website of the Chicago Manual of Style, which advises, It's best to be more straightforward and specific.
For example, use bullets or boldface zeros (••• or 000) to stand in for page numbers that cannot be determined until a manuscript is paginated as a book (but see paragraph 2.37 in CMOS).