The proposal’s technical attachment does contain a reasonable de-orbiting plan for the satellites, involving reducing the perigee to around 300 km which would result in a fairly rapid re-entry. The problem is that guidelines about time to removal (including the remarkably arbitrary 25 year recommendation) are just that: guidelines. There is no real international agreement about this either. Satellite manufacturers currently do little more than pay lip service to debris mitigation, and will use the cheapest, untested debris removal technology they can, with little expectation that it will actually work. Beyond an altitude of 600-800 km (depending on solar activity levels etc.) solar radiation pressure overtakes atmospheric drag as the dominant force acting on a satellite. SRP generates tiny forces which tend not to be applied in a way likely to accelerate deorbiting. The satellites as described in this article are likely to have a ballistic coefficient which will leave them in orbit for hundreds, if not thousands of years in the very likely case that their end-of-life manoeuvre fails. There just isn’t an incentive for Space-X to make it reliable.