Ok, I think we have some misunderstanding somewhere. Are you thinking you should be able to select the text on an HTML page? No, you can't do that. The SVG file which accompanies the HTML file you shared, is properly ungrouped, and I can select all the text
in the SVG file, in Inkscape. But you can't select it on the webpage. It's possible there's some way to do that, but I have no idea what it would be. That would be at least as advanced as the code on the CSS TRICKS article, and maybe more advanced -- if it's possible at all.
As I said, I know little to nothing about screen reading or screen readers. I'm not sure whether the text would need to be ungrouped in the SVG file, for the screen reader to able to read it, or not. Maybe not, maybe so.
When you put the SVG file on the html page, did you try to read it with the screenreader?
Also, the "20-bits" text above "RSTK" in the diagram is missing
I don't see any text above "RSTK".
When you're trying to select the text on the webpage, how are you doing that? Does the screen reader software allow you to do that? Is that what you're trying to do? Oh I have no idea about that. And I'm not sure who might know anything about that, within our community. I'm guessing the best place to ask for help with this is going to be the mailing list. The User mailing list. You can find the link to subscribe here:
https://inkscape.org/community/mailing-lists/Of course we can keep stumbling around here, if you want to try. I can spend some time reading the CSS TRICKS article more closely, and maybe come up with some clues. But I'm starting to think you're trying to do something that is too far over both our heads, at the moment. If someone on the mailing list knows something about using inkscape with screen readers, that would be your best bet. But there might not be anyone who knows. This is the first time I've heard anything about this.
If no one on the mailing list knows, and if the screen reader software is free, I wouldn't mind installing it, and trying to experiment alongside with you. Or if you don't want to try the mailing list yet, I'm happy to stumble around some more. Just let me know.
Edit
The SVG file which accompanies the html page, is different from the SVG file you attached earlier. In that one, I do see the "20-bits". The reason it doesn't show up is because it's made of Flowed Text. I think I mentioned about that before, but flowed text doesn't show up anywhere outside of Inkscape.
In Inkscape, select the flowed text. For future reference, notice in the status bar, how it says "Flowed Text....". Now Text menu > Convert to Text. When you do that, you'll see the text moves over slightly. You can just move it back, using the Selection tool. And now it will show up just like all the rest of the text.
Edit #2
Possibly having removed the flowed text from the file will make it work with the screen reader? If not, there is another flowed text object in the file. It's empty, meaning there's not actually any text. But I'm thinking it still might confuse the screen reader (keeping in mind that I have no idea how they work). So we could try removing that one as well, and cross fingers.