Toi
I have never heard it presented that way. It doesnt sound that way from the MEchaber “צְרִיכָה לִהְיוֹת זְקוּפָה” (not that its passul horizontal, but that it has to be vertical) .
Pischei Teshuva “והגר״א זצ״ל בש״ע שלו פסק כרש״י שצריכה להיות זקופה ע״ש וכן הוא בהנהגות שלו”
while your explantion makes a lot of sense, is it your own or have you seen it somewhere?