RTP and RTMP are totally different protocols and the blog shows how to dump to HLS, not MP4 
Membrane can dump both RTP and RTMP to HLS, while dumping to MP4 requires transcoding. Dumping to MP4 without transcoding is not really straightforward, but support for that is coming too: Opus payloader and allowing in-band parameters with H264 Payloads by daniel-jodlos · Pull Request #38 · membraneframework/membrane_mp4_plugin · GitHub.
