So then how is there any proof from this that we say migo in the sense of actual effectiveness for the usual sense of migo in “one who lifts a lost item for his fellow”?

In the case of lifting a lost item for his fellow, the issue is whether he can acquire it for him without an actual migo, but only by the ordinary law of agency/acquiring on another’s behalf. And that is exactly what is rejected here: that in this case it works by virtue of an actual migo, so there is no proof.

There is a Talmudic passage later on, on 9b, that says:

We learned there: If someone gathered the pe'ah and said, “This is for poor so-and-so,” Rabbi Eliezer says: He acquired it for him, and the Rabbis say: He should give it to the first poor person he finds. Ulla said in the name of Rabbi Yehoshua ben Levi: The dispute is when a rich person does so for a poor person, for Rabbi Eliezer holds: since, if he wanted, he could declare his property ownerless and become poor and then it would be fit for him, now too it is fit for him; and since he acquires for himself, he also acquires for his fellow.

This implies that there is no need for an actual migo in order to acquire for one’s fellow in the case of a lost item or pe'ah; a potential migo is enough, meaning he could have. If so, why does the Talmud compare the case of one who lifts a lost item for his fellow, where it takes effect because of “since he acquires for himself” potentially, to the case of partners who stole, where the acquisitions of theft take effect because of “since he acquires” actually?

I thought to suggest an explanation that even in the case of partners who stole, we are dealing with a potential migo, but the condition for the validity of the migo is willingness to actualize it. In the case of a regular agent for theft, the agent does not want to steal for himself, so there is no “since he acquires for himself” here. But in the case of partners, he is willing to steal for himself his partner’s half, and therefore there is “since he acquires for himself” here as well, potentially and not actually.

In the case of a lost item too, it is an actual “since he acquires.” Each one lifted half, and since his lifting is effective for himself, it is also effective for his fellow.
The passage you brought here is “since he is able to acquire,” not “since he acquires” — that is, migo in the conventional potential sense.