Your iframe URL starts with //html5-player.libsyn.com whereas you added the domain libsyn.com.
Adding a domain in the allowed iframes doesn’t automatically allow the related subdomains.
The protocol is also important.
Adding https://html5-player.libsyn.com won’t work because your iframe URL starts with // without specifying the protocol.
In order to properly allow this iframe to be embedded, you need to allow the following iframe:
//html5-player.libsyn.com
The second issue is that your iframe has the attribute width="100%". The attribute width only accepts numerical values (that will be rendered as pixels, not percentage or another unit).
If you want the iframe to use the full post width, set a high numerical value like width="1000" without specifying any unit.
In brief:
Add this allowed domain: //html5-player.libsyn.com
In the iframe attributes, replace width="100%" with width="1000"