I think the comment made by some is misleading
I can't say I noticed any of my apps over the last year that are not full screen, there must be some odd reason that in a few reviews that showed some popular apps were not going full screen, but I can't say I have ever really noticed any that do not fill the screen
All my little 3rd party utilities go full screen as do other popular apps like Ring etc always have
So I am not sure whats driving this observation on a few social media apps as all others seem fine on my Fold V1
It's not like I have ever had the small Iphone app on an Ipad experience