What's this mishna doing here? (Eruvin 59a)

When I started learning the mishna on 59a I assumed it was talking about eruvei techumin. Took me a couple minutes to realize that we had suddenly switched back to eruv chatzeros. Then I looked at the next mishna and we switch right back to eruvei techumin. That leads to the obvious question: What is this mishna doing in the middle of a perek all about eruvei techumin??

The Keren Orah pointed me to the Tosafos Chadashim in the mishnayos who asks this question. They answer that the last mishna concluded with the line that we're lenient by eruvin so this mishna shows that is not only true by eruvei techumin but even by eruvei chatzeros as evidenced by the fact that you can make an eruv around a city of a yachid that becomes a city of rabim. Not crazy about the answer but I haven't seen anything better.