Part 1
For the benefit of the others who mentioned the same issue (and because I think AppSumo should have been transparent about this in the first place), I am posting the email reply AppSumo sent me:

"TidyCal Tier 1 Plus plan was given to all Plus members as a Freebie Plus Exclusive plan, and unfortunately, those plans cannot be upgraded."

Part 2

"Those plans have been carefully crafted only to cater to our AppSumo Plus members.
And though they look the same as any other Tier 1 plan, they are different as they are dependent on your Plus status and cannot be upgraded.
This license is connected to your Plus membership, which means that if your Plus membership gets canceled one day, the free app will be gone too, unfortunately."

Part 3

So while Plus Exclusive freebies are listed as "LTDs", they are actually not. They are tied to your AS Plus subscription and you will LOSE ACCESS as soon as Plus expires. I.e. you don't own it, you are renting it with rent due every year. AppSumo: that's not an "LTD", it's a membership benefit or a "hook" depending on how you look at it. You should have been transparent about it.