Everything horses do is a form of communication, as they have no words, so rely totally on body language and sounds.
Horses are experts on interpreting other horses. So if she is concerned by something and snorts, another horse will note that she has seen or sensed something she is wary of, depending on the context.
Snorting can be a reaction to fear, an unfavourable sensation, or clearing the nostrils. It's arguable that snorting can be associated with positive or negative emotions. So it's important when interpreting any type of horse behaviour to take in the whole context of body language and the situation/environment.
A horse that comes across something it fears, like a strange dog, may approach the dog with head high, neck arched, high on the toes and snort. This is part of an aggressive type of snorting, that encompasses flared nostrils. As you mentioned, it is theorised this type of snorting is a response to a release in adrenaline.
When rolling or eating a horse may get dust up their nose and the snort will be literally to clear out the nose. The horse may be perfectly content.
So taken in context, other horses will definitely take heed of a warning snort. As they rely on one another to stay safe as a herd.
It's also noteworthy that snorting is similar to blowing, but there's different ideas about why horses do this.