I thought Mediatek also did their own modem+cpu combos?

They do. And so does Samsung. I haven't checked but I would be surprised if Huawei didn't have their own modems too.

Yes Huawei too with the Balong modems, through their HiSilicon subsidiary: http://www.hisilicon.com/en/Products/ProductList/Balong

In a few years it could be so that all the tier 1 smartphone vendors have their in-house cellular modem.