And then the admins decide what gets deleted and what is allowed to remain.
Not even that's the case. Wikipedia blocks shared IPs. They even blocked the National Library of Scotland which has an official Wikipedian.
Then use a different IP or register an account?
Almost anyone can edit it though they seem to have blocked Larry Sanger the co-founder for being a pain.
Most of the IPs are used are blocked before they even start. If you do get in there, an editor without a clue will challenge your edits even if he (nearly always a "he") knows nothing about the given subject.