Is a president that also holds the position of treasurer an example of a conflict of interest ?

In your question, it should be noted that this question could refer to a president of a country or a president of group or organization. In either case, it may or may not represent a conflict of interest if the president holds both positions.
A critical factor will depend on the structure of the organization. If there are limits to what the president can do with the money, there shouldn’t be a conflict of interest. For example, if the governing body or board of directors has to approve all expenditures, there should be no problem with the president also being the treasurer. Additionally, if there is a yearly audit, any issues with spending money should be caught in the audit. If these checks on the president’s power regarding finances don’t exist, then there could be a conflict of interest. Without these checks, the president would be able to spend money on anything he or she wanted to spend it on, which might lead to personal gain or to harming the government or organization.
I have seen situations where a top leader is also the treasurer, and there has been no issues or conflict of interest.