Turkey says they're defending against a terrorist PKK insurgency with arms and support coming from Northern Syria.
NATO officially recognizes PKK as a terrorist group and despite the disclaimers, the Turkish PKK and Syrian SDF have close ties, a shared ideology, and similar aspirations.
Turkey warned the US not to provide heavy weapons to the SDF, but they did so anyway under the guise of fighting ISIS and now some of those weapons and fighters have wound up in Turkey.
So the conditions already exist on the ground for Turkey to invoke Article-5 if it wanted to, and the Turkish incursion into Syria to stamp out the cross border insurgency has the imprimatur of NATO acceptance.
Which is not to say Turkey isn't a bad actor (they clearly are), but as a member of NATO they are mostly immune from Western interference, while still being able to call on them as needed.