What the response means is that we generally don't migrate questions to beta sites at all, unless the question is already distinctly off-topic for Super User and specifically on-topic for that beta site.
New sites need to grow on their own accord. Poaching an established site is not an effective way to get new users. Furthermore, it is a very confusing user experience for the question asker when their question which was on topic is suddenly moved to a new site they've never seen before and may have no interest in participating in.
On Super User, we frown on migrating questions away if they're offon-topic here. Flags requesting that questions be migrated to the Raspberry Pi site will be declined if the question is on-topic for Super User.
That being said, if you see a Raspberry Pi question that has been around for a while (like, several weeks) and has no answers, it's fine to leave a polite comment letting the question asker know that there's a site for that subject. But leave it up to them to request the migration - it's their choice to make.