My interpretation of that (and I’m no expert!) would be:
Plex tries direct play, which essentially means transmitting the file “as is” with no recoding and no remuxing. The profile now accepts http/mkv/hevc for direct play, but not http/mkv/hevc/dca. DCA is the formal name for what we normally call DTS, so the file contains DTS audio and the profile doesn’t allow DTS audio to be passed through as is. I don’t think the Chromecast supports DTS directly, so disabling direct play is probably correct.
Then Plex tries “direct stream”, AKA remuxing – it should be able to keep the video part of the stream as it is, but still transcode the DTS audio to for example AAC, which is much cheaper than video transcoding. Then it should repackage this combination into MP4 or MKV. But while it allows HEVC in direct play, it doesn’t want to keep HEVC when remuxing for some reason, and then falls back to H264.
This might be controlled by another part of the XML file, in TranscodeTargets. Speculation: If you add another VideoProfile there to allow HEVC as a transcode target, maybe it will allow HEVC to be remuxed rather than transcoded? It seems strange that TranscodeTargets would affect what does NOT have to be transcoded, but who knows…
Or maybe you’ve already done that? Could you post the complete profile as it looks now?