Maybe because they are using 21st Century English, not whatever centuries-old version you are stuck on.
“Thru” has been a valid alternative spelling of “through” for many years now. (Except maybe to old-maid prissy english teachers.)
For me, it was settled when Adm. Grace Hopper specified thru and through as equivalent alternatives in the COBOL language definition, circa 1957.
Of course, you are welcome to follow whatever ‘authorities’ you wish. But don’t expect the rest of us to follow you into the past.