This appears to be a "shot over the bow of the ship". Mint whose popularity has exceeded Ubuntu is making a subtle statement which could be signal about Ubuntu. It is seen in this announcement from them.
Or, it could be their way of attracting an even larger community of users who might be considering use of Ubuntu by inferring its demise.
FYI
The Mint team has been "burying" Ubuntu for five years or more 
And just in case, they keep a backup LMDE in the basement.
Clarity, aren't you confused or scared by this 
:
do not use Yumi or multiboot technologies with LMDE as it won't install properly.
Different devices. Different approach.
... Yumi or multiboot technologies ...
Not really.
I think the user-friendly approaches we are using to boot ISO files, now in the forum, will only become ubiquitous over the future. Mint will insure theirs works for future vs. taking any steps to inhibit use.
Mint considers itself user-friendly and a user-assist technology.
Over the past couple years, Tovalds and others have shared their concerns/views of Linux, in some form, as a desktop system. All the while, steady improvements from manufacturers along with Developer upticks has rapidly moved forward with the likes of Wayland, Pipewire, LVFS, and kernels rewrites that has changed what we knew of Linux to what manufacturers have envisioned in their product use.
Chip shortages, currently, will slow things a little, but as the supply chain opens, I believe something very interesting is in store. Autonomous existence is upon us.
OK, all kidding aside, the irony of that... Here's is something which is really of interest for Linux desktops use/users: "Fedora ..." If it goes GA with the stability in the packages shown in its current 'Rawhide version's" packaging, it will surpass every desktop distro in the wild, today!...yes ...EVERY!
This bit of information just dropped as it looks at Fedora just as it is set for release in less than 30 days.
FYI
P.S. Until now, I haven't used Fedora since its very 1st release years ago, when it got its name as the development release.