I believe the intention is that a Master Construct may be extended via Preserve Duration or Permanent Duration, but no other rituals may be cast on it. The language is a little disordered, but in bullet points:
1. A Master Construct is a valid target for an extender ritual, i.e. Preserve or Permanent Duration.
2. The difficulty of an extender ritual is calculated based on the strength of the Master Construct (see the quoted chart).
3. Despite the chart saying "15 point Golems are counted as 5-8 Rituals for purpose of calculating Preserve or Permanence difficulty", it should perhaps say "15 point Golems are counted as 5-8 Rituals only for the purpose of calculating Preserve or Permanence difficulty.
4. After establishing that for the purpose of calculating extender difficulty, and only for that purpose, refer to "All Master Constructs, once completed, are considered 20 Ritual Effect Items."
Essentially a golem is a 20 rit item and therefore not a valid target for any other rituals. An exception is made for extenders, and only for extenders. Since extenders calculate difficulty off of the number of rits on the extended item, the chart exists so that it is easier to extend a 15 point Master Construct than a 60 point one.