Does it looks like a bandpass matching network?
L1 (27 nH) and C1 (5 pF) form a band pass filter whose resonant frequency is: -
$$\dfrac{1}{2\pi\sqrt{LC}} = \text{433.16 MHz}$$
This is near enough to 433.92 MHz not to get in a pickle over because it's a fairly low Q filter due to the impedance of the antenna assumed to be around 50 ohms: -

Picture taken from this on-line analyser.
C2 and L2 form a high pass filter like this: -

Picture from this analyser.
I've chosen R (in this example) to reflect the 50 ohms output impedance of the previous band-pass stage.
C2 and L2 are an impedance matching stage and, for it to be effective necessitates that the antenna pin on the SYN531 has an input impedance of several hundred ohms but unfortunately, the data sheet is sadly lacking in this respect so you have to take their word for it.
Anyway, the impedance transformer works like this: -

This means that: -
$$R_L = \dfrac{L}{R_{IN}\cdot C}= \text{178 ohms}$$
I expect that the input impedance of the SY531 is a bit more than this and, if we use the other equation we find that &quot;F&quot; is 538 MHz and pretty much where the peak is in the 2nd simulation graph. So, I expect the input impedance of the SY531 will be around 200 to 300 ohms, hence it uses an impedance matching circuit.