No, it is not from -아/어서. -아/어서 only applies a verb or adjective, not a noun like 혼자.
In this case, 서 is a special particle that attaches to a subject showing number. It indicates the number in relation to the action.
- 혼자서 그걸 다 먹었어? Did you eat all that by yourself?
- 이 작업은 두 사람이서 [or 둘이서] 해아만 한다. It takes two people to work on this task.
(둘 means two, and 이 is added if the noun/number ends in a consonant, as in 셋이서 = in/by three)
- 고등학교 동창생 넷이서 모였다 = Four high school classmates got together.
혼자서도 has another particle, 도, appended to 서 to add the sense of "even".
So it means "even alone" or "even by yourself".
- 혼자서도 되겠어요? = Could you manage even by yourself?
- 혼자서는 못해요. = One person cannot do it. (는 adds emphasis, like "just one", especially in a negative phrase)
- 둘이서 이 큰일을 할 수 있을까? = Can just two people handle this big task?
서 can be hard to master as it appears in many different constructs. It can be a contracted form of the particle 에서 (at, in, from), the verb ending -아/어서 (because) with 아/어 merged into another syllable , or a particle in its own right as in your example.