Probably compatible in the sense that it can be made to work with a RPi but not plug and play. I've never used one of these SIM900 things so cannot advise. I do recall some posts on the forum. Have a search here and on the internet generally and see what you can find.
So firstly there seems to be guidance on how to use modules like or similar to the one you're looking at. Secondly it would appear that there are SIM900 modules designed specifically for the Raspberry Pi, and it might be prudent to get one of those.
The purpose of a little toe is to ensure you keep your furniture in the right place.
The only concession they have seemed to have done is to add jumpers so that the serial RX signal is resistor divided so it doesn't put 5V on the RxD pin of the Raspberry PI, and they made the board size such that it did fit on the original PI model 1, but not on the later models (except perhaps the zero).
That original link seems to point to a module designed and intended for an Arduino with "Raspberry Pi" probably added to attract attention. No idea what "E9U6" references.
If you know what you are doing you can probably get it working with a Pi. That you are asking suggests that you don't and it would probably be better to get something designed and intended for the Pi, has details of how to configure and use it with a Pi.