If there was a bug where the ubuntu website could force your browser to display https:// and serve content over http without a warning you would have most definitely heard about it before now.
As to why most mirrors serve over http, it's for the same reasons presented here: https://whydoesaptnotusehttps.com/
I believe your link does not explain why Ubuntu’s .iso files are not on https, as one of its justifications is that APT uses other mechanisms for integrity checking, which doesn’t apply to OS installation disk images.
Also grandparent's link states
>HTTPS does not provide meaningful privacy for obtaining packages. As an eavesdropper can usually see which hosts you are contacting
Which is untrue. An attacker can't tell which specific packages are being downloaded, which could be important depending on your threat model.
(The argument can be made size of DL leaks info, but that's a lot of work, and multiple packages could have same size)