This show is officially, canonically, set in the "prime universe", aka it is in continuity with ENT, TOS, TNG, DS9, VOY and the TOS & TNG movies. Contrasted to the "Kelvin universe", which is the continuity of the 3 nuTrek movies (and potentially ENT, although this part is debatable after the "effects backward in time"- approach of Beyond).
These are the two timelines that officially exist. And DIS is part of the former, not the latter.
Every fan is of course allowed to have his own interpretation and/or headcanon. Like @BillJ 's approach, where TMP and ENT already were reboots. However, those are fanon theories, and different from official canon.
The point is settled.
Can we please move the "I think DIS is a reboot"-theories and arguments to their respective fan interpretation-threats? And not try to argue facts?
Or maybe change the threat-title to "THEORY: Discovery is set in the Kelvin timeline/ a reboot even though not confirmed"?
I have a better idea. Why don't you just quote the relevant statements made by TPTB which unequivocally state that DSC takes place in the prime timeline?