From the Waiver Request linked from the article in the question:
The Boeing 702HPspacecraft design features a reaction wheel pointing control system. As all venting must occur within the capabilities of this pointing control system, the firing of thrusters for venting and to address momentum buildup must be of more limited duration and additional time is needed for the pointing control system to respond. Other spacecraft of similar design have required two to three months of continuous operations to fully deplete their bipropellant systems.
and in fact, very little time may be available for venting:
In the absence of additional ground station solutions, Spaceway-1 will first need to increase its eastward drift before turning around and completing a near-continuous burn until it reaches its disposal orbit. This sequence will take approximately 21 days,leaving at least seven days for venting operations before the spacecraft is to be decommissioned.
In addition some of the propellant can't be vented:
Spaceway-1 is a Boeing 702HP that is not designed to vent the pressurant used during orbit raising, which was permanently isolated from the propulsion system by firing a pyrotechnic valve at beginning of in-orbit life. Information regarding the approximate amount of residual helium and xenon gas that cannot be vented at the end of operational life is as follows: HE1: 0.5096 kg, HE2: 0.5096 kg, XE1: 5.05 kg, XE2: 5.05kg
But the whole thing seems a little confusing as the satellite has two propulsion systems
- Moog Columbium 22N MMH/MON thrusters, used for orbital insertion
- Boeing XIPS-25 165mN xenon ion thrusters used for station keeping