Idlib used to be an Al-Qaeda stronghold before Turkey took control of the area. Al-Nusra operated as Al Qaeda in Syria and was supported by Turkey. This looks bad for Erdogan if Al-Nusra was hiding Al-Baghdadi.
2017: 
"In April 2013, Baghdadi declared that the Nusra Front had joined forces with his group to become the group known as ISIS. The Nusra Front denied such a merger, and distanced themselves from ISIS’s attempt at gaining a foothold in Syria" time.com/4428696/nusra-…
2013: "Baghdadi traveled from Iraq to a town in Syria’s Aleppo province, where he was joined by Arab and foreign jihadis who had formerly fought for Golani’s Nusra." reuters.com/article/us-syr…
Erdogan: “They say [in the West] that PYD and YPG are supported because they are fighting against DAESH. That is a big lie. Al Nusra is also fighting against Daesh. Why do you say they are bad?” he reportedly said. aei.org/foreign-and-de…
2015: Turkish intelligence helped ship arms to Syrian Islamist rebel areas "At the time of the searches, the Syrian side of the border in Hatay province, which neighbors Adana, was controlled by hardline Islamist rebel group Ahrar al-Sham [AQ/Nusra]." reuters.com/article/us-mid…
In 2014, western allies pressured Turkey to designate Al-Nusra Front as a terrorist organization, but "Turkey was a bit cool towards that, assuming at the time it was a force that was effective against Bashar al-Assad’s army" voanews.com/world-news/mid…
"The Islamic State’s network in Turkey is built on an older and well-established network of Turkish Salafists." voanews.com/world-news/mid…
"A small subset of this population has links to international jihad and, for about three years after the beginning of Syria’s civil war, operated relatively openly in numerous Turkish cities despite being under surveillance by Turkish intelligence for suspected links to al Qaeda"
