As I am sure that you know, one of the meanings of "sath" is level. For example:
This book has been written to teach in high school level.
این کتاب برای تدریس در سطح دبیرستان نوشته شده است
As a native speaker, the sentence you mentioned conveys the meaning that, as my other friends have said, the deputies of foreign ministers of seven countries attended to the conference. but How is that?
I am not confident that what I am going to say is really true, since I am not a specialist in linguistics, nevertheless, it makes sense.
You know that when the president of a country attends an international political meeting, it conveys that the meeting is so important for that country. If the foreign minister attends, it is more or less important. If the deputies of foreign minister attends, it is not so important and so on.
You cam consider it this way. the meeting is important up to a level that the president personally must attend. So, when the president of a country attends to a meeting, it means that country values it so high up to president level. Thus, in Farsi, it make, sense to say " attended to president level" or
در سطح رئیس جمهور شرکت کرد
I hope it can be a help
