Very legit point! That is a concern that some of our prospective users do share. What they do in that case is vouch for ones who they know are looking out or can benefit from vouching (eg. folks who struggle at DS/algo interviews etc).
On the "no recruiter mails", I just realised that it might be confusing. What I meant that just because you vouch for someone, it doesn't mean that they will get bombarded with spammy recruiter mails. I think that means object is the person you are voucher for and subject is contingency and in-house recruiters at tech companies.