Yes, in the email I received from a department member, the terminal date for admitted candidates to accept is the start of April. But perhaps they are mistaken – where on the MIT Arch website did you see the notification date as April 1?
Not sure if you are asking about MA or PhD, but I was notified via email I was in a high waitlist position for the SMArchS HTC MA. The last date for admitted students to accept is April 1st, so it sounds like all acceptances have gone out by now.