This section of the guide does not apply to the 2006 A1186 (EMC 2113) MacPro 1,1. In this model (that I'm trying to disassemble in front of me, now), the heat sink cover is partly under the RAM cage, so it cannot come out before the RAM cage. The linked guide above seems to reference this fact.
Is there a way to submit an alternative version of this section for the models in question?