Ryu knew what the problem was.
Soul Techniques were different in that they didn't have the same limitations as Body Realm and Qi Realm techniques. In fact, their restrictions are even looser than Body Realm techniques as well.
If Ryu was so inclined, he could use even a Chaos Grade Soul Technique. But whether or not he could bring out its strength and comprehend it was a different matter entirely.
[New Phase Golden Moon] was ultimately a Transcended Grade Dao Charm Soul Technique. This meant that the Dao that underlied it was the equivalent of a Transcended Grade Dao as well.
Right now, Ryu was so far away from that level that it was impossible to fathom the gap. The fact he managed to finish reading the technique at all was a testament to just the level of talent he had to begin with.