The MGMT show is a headlining show. It is really not that surprising in that they sold out the 9:30 last time and IMP commonly puts band at Merriweather in the summer that aren't quite big enough for the venue.
Personally I think it is great as the venue is very comfortable when it is not really full (easy parking, short lines for concessions, etc.) and it is a much better venue for rock shows than DAR.
Also, the Ticketfly system worked just fine this morning for the presale.