Is the curse of Ymir ever proven to be real ? (I'm a new viewer of the show and have reached S4, partially spoiled)

I feel like the many lies Marley spread about Ymir, I feel that the curse of Ymir is also a rumor/lie.

I hypothesise that it's to prevent any betrayal from the people who possess the power of the titan. Hence they used the myth of Ymir to ensure that they have all the ppl with the titan powers under their control.

Basically, being on the war front for a long time makes people more and more aware about the war crimes Marley have done, and you don't want people with titan abilities to betray you.

That's what I feel. Can anyone confirm this ?