Being blunt, that's primarily Erdogan's propaganda. The actual nature of links is hard to tell, and both fight IS together, but they are not the same. To use just one of many articles, you can glean the sheer complexity of the nature of their relations. The US makes the distinction between the YPG and the PKK; Erdogan does not and so he bombs both.
The Kurdish situation is complex, but effectively while authorities (and by this I mean Mahmoud Barzani) in Iraqi Kurdistan have deliberately distanced themselves from the PKK, those in Rojava have not, or at least not to the extent that Erdogan wants. Barzani treads a cautious line; the YPG don't, in part since they need all the help they can get against IS. Hence Erdogan has little qualms about treating them as the same.
Erdogan also does not want the entire Southern border to become Kurdish, and given that they are basically the only ones having significant military success against IS, this would have been likely... unless they are weakened. Hence the ideas of buffer zones (ie, Turkish dominated regions, not Kurdish), not allowing supplies to help the YPG and so forth. Its his 'Kurdish Strategy'.
Also, how neat is it to divide the Kurds? If you can split the the Iraqi Kurds from the PKK and the YPG, they can't support each other, and they certainly can't go and support the Kurds in Turkey, or beat back IS, or form a single state. People who don't help the PKK lose legitimacy in the eyes of the Kurds, while those that do you can target. Meanwhile the West doesn't know who to support, and so loses interest. Divide and rule.
Not that even if they were I'd want Erdogan bombing them.
Agreed.


