Reddit also uses commonmark, plus some GitHub flavored markdown extensions, via the comrak library, plus some custom extensions. So Reddit markdown should mostly look like GitHub.
The second [1] is somebody that posted a ``` block but also indented all lines with four spaces. That renders as a code block with ``` lines before and after it on old Reddit, and as extra indented code on new Reddit.
If you use RES you can look at the raw Markdown code for each comment using the source link.
Maybe you´re thinking of inline code which use a single ` before and after the code content?
Screenshots: https://imgur.com/a/TKSKoEB
[0] https://www.reddit.com/r/cpp/comments/e16s1m/what_is_wrong_w...
[1] https://www.reddit.com/r/cpp/comments/e0fdp3/simple_timer_fo...
The old Reddit formatting wiki page doesn't mention ``` now I look, there's a note on new Reddit about sticking to spaces for compatability: https://www.reddit.com/wiki/markdown Would be nice if they fixed it indeed.