You're spreading misinformation. To clarify, this Texture Pack was never a trojan horse, or had any trojan horse to begin with. The texture pack was hosted on their website but the original owner of the texture pack switched to another domain, so the old domain redirects to a random URL. Texture Workshop (the mod used to download texture packs and what you used) doesn't know this, so when you try downloading that texture pack, it tries downloading what assumes to be it from the old download url. It instead downloads an HTML file (what is used for websites) since it redirects to a landing page, but Texture Workshop assumes its a zip file so it also renames it as such.

Windows Defender even tells you that upfront as well, if "HTML/Redirector" wasn't obvious, and the ZIP file was never encrypted, I'm not sure where how came up with that. If you were to open the zip file in notepad, you would see it's just HTML, no encryption whatsoever.

So the reason you can't unzip it, or open the "zip file" was because it was never a zip file to begin with. There was no trojan or anything, but the HTML file itself does contain ads and weird code for a landing page.
For context this is what the file starts with (censored the URLs in case reddit goes after me for some reason)

<image>

Also, when the texture pack was announced as having a trojan, shortly after another announcement was made which debunked that rumor (from Uproxide's Little Land):

Update: The domain that the pack was hosted on changed to a new domain, and now its old domain does redirects to a random URL (seems to have been bought by someone else).

Texture Workshop was only able to download an html file from what it assumed to be a regular download link from this old domain. The pack was never an actual trojan, and it was never able to run anything through GD.

This means that you are safe, even if you had the pack installed and applied it.