If you want to find an official site for a Ubuntu project, don’t ask a search engine, but go to ubuntu.com (under the control of Canonical) which will send you to the following page listing the official web sites for all Ubuntu flavors
You’ll then go to lubuntu.me for Lubuntu.
The site you went to, is not under our control, nor under the control of Ubuntu/Canonical.
Did you confirm your checksums of the downloaded ISO from a Ubuntu/Lubuntu site to confirm your ISO was valid? (I’d do it given you downloaded from an unofficial site). Many sites do in fact point to ubuntu.com to download, but better safe than sorry.
Yes you can force the 19.10 upgrade if you wish, and no I wouldn’t expect issues unless they were documented in the release notes (https://lubuntu.me/focal-released/).
If it was me though given you may not have used an official ISO I’d consider a re-install unless you verified your ISO using checksums from an official site, and use official Ubuntu/Lubuntu sites in future.