Hi Wales,
Hope all is going well.
To answer your question very briefly, Sikhi is 100% monothestic.
In Jap Ji Sahib and other prayers, there are various references to devi devtas. You are quite correct in that they are often used to illusrate some of the different facets of the power of God.
The passage which you quoted actually rejects the idea of the supremacy of the deities if you examine it carefully.
The passage speaks of three elements of the power of God, Creative, sustaining and destructive. These are describes as Brahma, Vishnu and Shiva; three of the most recognised Hindu deities. The passage is clearly not confirming the status of these deities and the split of powers between them. Instead, it indicates that even if there were such deities, there is a power above them which sees them and controls them, but they cannot even see this one entity which is above them (ie God). This is therefore confirmation that there is one supreme master rather than a multitude which have different powers.
Hope this clarifies somewhat. I apologise for not having the time to go into the level of detail the question requires to be answered properly.