AFAIK, your answer is correct. Azure Reserved VM Instances (RIs) are an Operational Expenditure (OpEx), not Capital Expenditure (CapEx).
Even though Reserved Instances require upfront payment (which might resemble a capital expense), they do not involve ownership of physical assets. Instead, they are a commitment to use Azure's infrastructure over a period (e.g., 1 or 3 years), which aligns with the pay-as-you-go, service-based model of OpEx.
CapEx typically applies when purchasing physical servers or data center equipment, where the cost is capitalized and depreciated over time. Since Azure RIs are just a discounted pricing model for cloud services rather than asset ownership, they remain OpEx.
If the above response helps answer your question, remember to "Accept Answer" so that others in the community facing similar issues can easily find the solution. Your contribution is highly appreciated.
hth
Marcin