DY: Presumably all of them would have previously disallowed bochurim in their yeshivos from dating, since their yeshivos are for younger bochurim.
The better answer is that the purpose of this ad was not to change the rule–that they could do by kloping on the bima. The purpose was for outsiders to know that bochurim in those yeshivos are open season.
However, it is all dumb. Because those yeshivos generally don’t have bochurim in their 4th year.