Looks like a photoshop to me. Reading this on a laptop, I tilted by screen back, and noticed a fuzzy grey pixelated border around the main text of the tweet – most noticeable on the blank space two lines below the word “justicia” . This unusual feature is absent in the genuine tweets I’ve seen, like the ones in the daily round-up. I’m no expert, but having used and seen the results of image editing software before, especially when the images are reproduced in low quality, I would theorize that someone copied and pasted the inflammatory comment onto a Twitter post template with Alonso’s user information.
(Of course, the other possibility is that the tweet is genuine, and the visual artifacts are the result of low image resolution, in which case my hypothesis would be incorrect.)
The above notwithstanding, Alonso should be, and in all likelihood is, intimately familiar with the niceties of F1 politics. I don’t believe he could have been so brash as to post such a thing.