That is not necessarily true. The official must judge whether the tackle was the result of the additional contact or the pull at the collar. Just because there is additional contact does NOT remove the possibility of the horse collar. This is straight from the NFHS pre-season presentation.
Respectfully, AB, and without the benefit of having seen the NFHS pre-season presentation, I have only Case Book 9.4.3 Sit M (b) to go on.
"A1 is running...B1 grabs A1's shoulder pad opening from behind and: (b) B2 comes in and tackles A1 while still in B1's grasp. Ruling: Legal".
AND, of course, how we have been told to interpret the rule.....
Is the NFHS pre-season presentation is available on-line or somewhere else?