I haven't seen any announcements about this but when searching fares saw that Southwest is selling nonstop tickets from Denver to Oakland. Has this been going on for long?

Example:

Outbound: DEN to OAK
Southwest #415
Feb 11
Depart 8:25 p.m.
Arrive 10:05 p.m.

Return: OAK to DEN
Southwest #173
Feb 18
Depart 7:00 a.m.
Arrive 10:30 a.m.

Total fare:
$158.60

Will UA and Frontier match?