The Russians insist they never entered Turkish airspace. I don’t think that is true, based upon the balance of information from both the Turks and the Americans. I also don’t think a 17 second incursion is cause for shooting down an aircraft.
What is "true" is based on whom you ask. That tiny little finger of land sticking down into Syria is not shown on Syrian maps of the region. The Syrians claim that territory, and have always claimed that territory as theirs.
Syrians hold the view that this land was illegally ceded to Turkey by France, the mandatory occupying power of Syria in the late 1930s. Syria still considers it an integral part of its own territory. Syrians call this land Liwa' aliskenderun (Arabic: لواء الاسكندرون‎) rather than the Turkish name of Hatay. Official Syrian maps still show Hatay as part of Syria
It is possible that the Russian pilot might have been under the impression that he was flying completely over Syrian land, and that he was unaware that Turkey laid claim to that 2 mile wide piece.