The frequency central supplies mentioned there expect 12 V AC, not DC. Creating ±12 V from laptop supplies is non-trivial; you need robust DC/DC converters, and the result still isn’t necessarily great (see the discussion of switched supplies in that thread for why). There are some designs out there that use industry standard converters to go from DC to ±DC, e.g the D112 that was discussed here, but at that point it might be easier (and cheaper) to use e.g. an RT-65B supply.
But I’d suggest taking the power supply discussion to that topic, to get more eyeballs on it.