Given that the consensus here has become "undergraduate questions are off-topic unless relating to graduate admissions", we need to support the proposed Undergraduates.SE so that these questions have a place to go.
The observation that "Undergraduate questions are off-topic here" does not lead to the conclusion that "We need to support the undergraduate SE proposal". This site can stand by itself. If there are enough individuals who are in favor of an Undergraduate SE site, the site will be created. The fact that we get a number of off-topic questions that happen to relate to a current site proposal is really not so relevant to any of us; if we want to support a new site, then we should sign up, if not, then not.
On that note, if you do not intend to support the site, please do not sign up. The last thing a new site needs is a community of uninvested invdividuals.
For what it's worth, I'm in favor of the idea of an Undergraduate site, but I do not think we should support it simply because lots of questions that are off-topic here would fit there. The point of the site is not to be a repository for our unwanted questions.
I think we need to be careful about what is meant by "we". I think the academia.se community would be well served that when closing off-topic undergrad questions that we mention the undergraduates.se proposal. I think we should do this whether or not we intend to follow/support the undergraduates.se proposal/community.
I for one will not be following or contributing to the undergraduates.se community, but I see no problem steering questions people that way.