False positive: js/prototype-pollution-utility

Hello, I'm getting a CodeQL alert for a Prototype-polluting function.
There is a guard in place (highlighted in image below):

Source code is here: https://github.com/mui/material-ui/blob/v4.x/packages/material-ui-utils/src/deepmerge.js#L10

Start with packages/material-ui-utils/src/deepmerge.js at line 10 and inspect the guard shown in the issue, then trace how the js/prototype-pollution-utility alert analyzes this pattern. Confirm whether the alert is a false positive and define the corresponding query behavior or regression coverage as the completion criteria.

Written by the indexing model from the issue text.