Technically, there are no combo boxes in HTML, because the <SELECT> can’t be typed into. See the Wikipedia article on Combo Box (emphasis mine):
A combo box is a commonly-used graphical user interface widget. It is a combination of a drop-down list or list box and a single-line textbox, allowing the user to either type a value directly into the control or choose from the list of existing options.
While the definition of drop-down list matches exactly what the HTML <SELECT> element does:
When activated, it displays (drops down) a list of values, from which the user may select one.
So your senior is right.