Although every word she says is true, it won't ruin your horse to play around at it. and, though it's best to have someone show you , you CAN learn a lot from videos and stumbling around on your own.
if your horse is already disprespectful and pushy, then doing it incorrectly will just confirm to the hrose that you are someone he can lean on and push on, so, there is that possibility. But, if you take some direction and are firm in what you ask, your results might not be 'pretty or perfect' but aren't likely to "ruin" your horse, either.
I respectfully disagree, I've had horses come into training because owners have successfully taught their horse to practically attack them in the round pen because they didn't know how to teach a horse anything.
It starts off small enough, maybe the handler is nagging with pressure and the horse pins an ear about it or the handler takes steps backwards instead of forwards. This escalates to maybe the horse pinning their ears and shaking their head at the handler, then kicking in that direction, then charging etc.
One such horse came in after breaking her owners collar bone, this horse had been inadvertently rewarded and had pressure released at all the wrong times till this horse charged her owner and bit her collar bone, breaking it. You could no longer get NEAR this horse on the ground without her trying to run you out of the pen. I had to work this mare from horseback for a solid week, doing everything. I pushed her into the round pen, did my round pen work, haltered her, etc. Honestly, this mare could have killed someone with the state she was in. By the end of 60 days, she was pretty much back to what the owner said she was before she tried to join up. However, to get her to this point was a lot of work with both her and the owner. The owner was the biggest problem, she was terrified of her horse and it took a lot of lessons, first with my horses to gain confidence and experience and then her own before she could safely handle her horse.
Is this kind of thing a common occurrence? Of course not, but this type of thing does happen when owners don't know what they are doing.