as salam alaikum
this opinion of some Maraji' is based on ahadith like the followings:
It has been reported that the Prophet, peace be upon him and his family, said: "There is no Riba between us and the people who fight us".
It has been reported that Imam Sadiq, peace be upon him, said: "There is no Riba between a Muslim and a Dhimmi".
On the other hand, we find hadith like:
It has been reportes that it was asked to Imam al-Baqir, peace be upon him, "Is there any Riba between me and the polytheists"? The Imam answered: "Yes".
All the above-mentioned ahadith can be found in the book "Wasa'il al-Shi'ah" by Shaykh Hurr al-'Amili.
Another evidence is given by the fact that taking the money from a non-Muslim with mutual consent is allowed by consensus; therefore the interest taken from a non-Muslims should fall in the same category.
However Riba is one of the major sins and it is categorically condemned in the Qur'an and sahih ahadith with no exceptions. That is why you may find ulama that have opted for a more precautionary stance not allowing Riba in its absolute sense.
With prayers for your success.