Right. I was saying that the fact that they chose to use «cela» means that the above sentence « Elle assume cela. » is not equivalent to the English phrase, "I suppose so." It is literally, "She assumes that," and I believe the French equivalent of "I suppose so," would be « Je suppose que oui. ».
The « e » in « c'est » is more open, kinda like the "e" in "bed;" in IPA, it is [ɛ]. The « e » in « cela » is pronounced differently depending on where the speaker is from, but it is typically pronounced as a more closed and rounded vowel; there is no equivalent in English, and in IPA it is [ø].