According to Google Translate, "Homeland is Racist" will translate to "الوطن عنصري", which you can compare to the graffiti:
From this article
These do look very similar. In fact, the only difference is the ي - or yā’. The written one is underlined, and the printed has two dots. This is likely a frequent difference, as there is no letter in the alphabet with a line instead of two dots. It is sometimes written with no dots, and sometimes with the dots so close they almost join.
However, online translators are often not great, and "Homeland" being the name of the show, is unlikely to translate well.
Translating back gives the following:
Google ⇒ "Home racist"
Babylon ⇒ "The racist"
Systranet ⇒ "The country is racial"
ImTranslator ⇒ "Home components"
This seems like it could be completely innocuous - there is plenty of racist graffiti, and this could just be saying that.
However, when I put the 4 outputs into Google, it gives completely different Arabic out, which seems odd. This does reinforce the case of it saying "Homeland is racist".
Originally, I thought that this was unlikely, but it does seem to be correct - this is saying the show is racist.
I don't know wether what the artists claim is true. This may have been put there on purpose as a joke, or may have been overlooked after rogue artists were given the contract. The latter seems a little unlikely, but there really isn't enough evidence to draw a conclusion.