Exactly. From a carbon/global warming sense, this is actually worse.
Plastic is carbon that never went into the atmosphere. It's actually nicely stored away, wildlife effects aside. Once you burn it, it's like you were burning oil.
Oil in the ground is also carbon that wasn't in the air.
If the amount of jetfuel consumed stays the same, the amount of CO2 going into the air stays about the same. Could slightly increase or decrease depending on the processes' efficiency compared to the marginal oil producers' efficiency.
Fuel made from recycled plastic will displace fuel made directly from petroleum. So it will make the first problem (disposing of used plastic) better while not making the second problem (carbon dioxide emissions) any worse.
Obviously this isn't useful if we manage to eliminate fossil fuels altogether, but we're a long way from that.
Plastic is carbon that never went into the atmosphere. It's actually nicely stored away, wildlife effects aside. Once you burn it, it's like you were burning oil.