The only possible, and I mean "possible", interaction between phosphates and an SWG would be calcium phosphate scaling. BUT, that can only happen when CH and pH are very high and phosphate levels are well above 25,000ppb. It's just something that really doesn't happen in reality.
But that's not what the SWG manufacturers are referring to. They are simply parroting the APSP and other industry organization that consider phosphates to be a contributing factor to algae growth, which is not really disputed. But, in a properly chlorinated pool, phosphates can be ignored as the kill rates far exceed the growth/reproduction rates of algae.