You mention 'the most prominent actions' but it should generally be singular. If there are several equally valid options such as picking an image from a gallery, a FAB shouldn't be used (guidelines mention this).
When you read between the lines, an important part is that the FAB is independent.
When you open gmail and calendar, both have a FAB that allows you to create a new item. But if you are in an item, there is no FAB. Similarly if you are in Maps, it shows a navigation button, but when you start typing it disappears and a similar-functioned button 'directions' button appears. In all three cases, the action is decoupled from any particular event/mail/location.
A filter is not technically decoupled from the items, but it's coupled equally to each item, so you could argue that's the same rule through a different lens.
Even more abstractly, the button has the single function of "create new [generic item]". Which includes "create new filter... but not modifying the filter. Which means that you should remove the button once you've made the initial filter. Or that every time you press the button you have to start create an entirely new one which means having to enter things like min-max price all over again. Neither solution is good for usability.
So taking a strict view, a filter is ambiguous in terms of neutrality and gives issues down the line.
In practicality though? They're guidelines, not rules. And people will intuitively understand that the button simply means 'filter' instead of 'make/edit'. So you can probably get away with it just fine - although it depends on the context and complexity of the overall interface.