Aren't the $15 fares only for DenDeal club members? F9 might be trying to ramp up club membership at this point.

NK is also offering crazy low fares out of PHL, and it's getting fare matched.

And, fare matching is going to happen. If F9 wants to be in PHL over ILG, ABE or ACY, and likewise NK for having some routes at PHL over ACY, it's inevitable that both carriers are going to be fare matched at PHL.

However, the fare matching seems sporadic. For example, AA/US hasn't fare matched every flight of it's day on PHL-CLT against F9 but just a couple of flights. And the low fares from AA/US often happen on days that are different than F9. e.g. F9 starts with a very low fare. A bunch of customers buy up the lowest bucket of fares, eventually to a point where F9's fares are just modest. Then AA/US discounts fares on the same day. In some instances, AA's fares are lower than F9's. If there are 3-4 flights between F9 and AA with low fares, the two might be able to co-exist in the summer with that. By Fall, leisure demand will drop, but F9 is banking on Florida.

WN is fare matching ATL-PHL and MCO-PHL, but often it's last flight of the day reaching PHL after midnight.

AA/US seems more aggressive fare matching better timed flights. However, since AA/US has high change fees, many bookings might be speculative anyways where pax end up deciding not to take the flight if they can't make it by the time the flight runs.