There are religious definitions/ rules of modest dress and then there are personal definitions of modest dress. For example, religious Jewish women cover their arms, wear high necklines, wear skirts below the knees and not tight clothing. Religiously for them, therefore, that is the definition of modest clothing. However, there is no expectation that this is a universal definition of modesty and no belief that women who are not Jewish should follow these rules.
A personal belief of what constitutes modest or immodest dress is a judgment about what for you seems to be modest or immodest. If I saw someone walking around a shopping mall in a skin tight micro mini skirt, stripper heels and the tiniest of bras I would personally consider them immodestly dressed. They themselves may not think that way, and that is their prerogative - we live in a free country. It would also not be right for me to comment on their clothes to them, it is their choice. It's disingenuous to suggest that we don;t make these value judgments every day.
The problem is only when we enforce these definitions and beliefs on others. If I say I am holding a modest clothing fashion event and then display a load of clothes that I feel is modest, then what is the problem? The only way I could see that as being offensive is if I then tried to force my views on everyone else - eg I said that my clothes are modest and women should only wear modest clothes, as per my definition of modest.
The people running this event are not forcing their views, their definitions of modesty on anyone. They are making an event targeted at a group of people who share the same views on what constitutes modesty as them. The event is not aimed at you, it is not aimed to force women to dress a certain way. It simply offers a fashion event aimed at people who share the same cultural view of modesty. It is you who are taking the line that by using the word that they have used to signal a shared set of values it must be a judgment against everyone who dresses differently. Nowhere have the advertisers suggested that.