I don’t believe that’s the case, unfortunately!
The tipoffs are:
【期間限定 無料お試し版】 in the title,
and on the listing under the buy button:
販売期限
2023/5/8 23:59まで
閲覧期限
2023/5/8 23:59まで
In other words, this listing is a “limited time, free-to-try edition”,
And it is on sale until 5/8 and after 5/8 also the time where you can view it ends (as opposed to listings that stop being on sale but if you bought them you can read them after).
So I would say these are the free-for-a-limited-time kind of free, not free to keep.
(if they were free to keep it would likely be expressed as a discount on the main series entry)
I’m never focused enough to trust I’ll make use of the【期間限定 無料お試し版】myself, to be honest! But they are a good resource to be aware of!