You are correct in your assessment.
URL.IsHighRisk, etc. are true/false depending on if the risk-level falls within those boundaries.
"URL.IsHighRisk equals true" would be equivalent to "URL.Reputation greater than or equals 50"
It's just easier to write a condition with the true/false than to do it by defining the numeric values.
The URL.ReputationString is the text literal of the risk levels. It is used mostly for logging, so the log entry would say "High Risk" instead of a number.
I think these are the current boundries:
Minimal Risk: <15
Unverified: 15 - 29
Medium Risk: 30 - 49
High Risk: >= 50