There are a number of possible interpretations on what 'bulking up' your usability tests with SUS scores might mean, so let's look at some of them.
Given that usability tests are generally much more in-depth and is generally in the context of specific goals or tasks, you can match the SUS score to add another data point to what your assumed user experience for this feature might be. If this SUS questionnaire is administered straight after the usability testing session as opposed to a pop-up that happens some time after the feature has been released, it provides a slightly different way of interpreting or comparing between the two results.
So the first interpretation of 'bulking up' the usability test is positive, which is to provide an additional data point to your overall picture about the user experience. And the second interpretation is weighing the number of responses you get from the SUS score to the number of usability tests and using that to influence your original usability test findings.
There is also this perception that the more tests you do, and the more data that you get, the more confidence you get become about the result (depending on the type of bias or biases you fall into), and I think this is probably the danger of trying to do more tests than it is necessary to come to a particular conclusion about a new feature. Often when trying to uncover serious design or usability issues, it should be easily identifiable when you have the right types of users, even with very small numbers. However, if you are trying to figure out some subtle detail about a feature, often the type of testing you do and who you put the tests in front of will determine the outcome.
But hopefully you can get a response from the person who made the comment originally to clarify. It is good to see that you are doing different types of testing, but just make sure that you are treating the results with the proper assumptions and interpreting them in the correct context when making design decisions.
