I've not opened one up, but yes I think they are wire wound resistors. As the power rating is not specified on the one you have ordered, I have no idea if it is adequate or not.
The PAW is not transmitting all the time so you may get away with the one ordered. For the cost of a few quid, I wouldn't risk it if you plan to use it long term. However if you are just doing a few tests, see if it does get warm, and make a mental note of the temp. If it changes much, either getting cold. or hotter, then stop using it.
You could always return it for the cost of postage?