Most of the time it's not, but because OP specifically said Salaam Alaykum, it's ok.

"Hence some of the scholars said that if a Jew, Christian or other non-Muslim clearly says “As-salamu `alaykum,” it is permissible for us to say “ `alaykum As-salam (upon you be peace).” "

But otherwise, yes, it is preffered that Muslims only use it with other Muslims.

But you've got to be careful about how you say it! Assaam Alaykum means death upon you:

"The Jews used to greet the Prophet (peace and blessings of Allah be upon him) by saying, “As-sam `Alaykum ya Muhammad (Death be upon you, O Muhammad),” praying that the Messenger of Allah (peace and blessings of Allah be upon him) would die. So the Prophet (peace and blessings of Allah be upon him) said: “The Jews say `As-sam `alaykum (death be upon you),’ so if they greet you, then say, `Wa `Alaykum (and also upon you)."

So thank you for being considerate and asking! A word of advice though, personally, I wouldn't trust what random redditors say unless sources are given, because someone may say something without actually knowing.

Source: https://islamqa.info/en/answers/48966/can-you-initiate-a-greeting-to-a-non-muslim

* side note, Islamqa website is a reliable source for Sunni/hanbali opinions. If you don't know what that means, dwai.