Yeah, it really is more like google play store or shopping websites and similiar apps/websites (although there are some that have a better design I guess). I’m not really a fan of it either, but I guess people being used to those (which is probably the majority of the userbase of flatpak) feel more comfortable with it.
My guess with the difference between “trending” and “popular” is that the former means lots of recent downloads and the latter a lot of downloads in a longer timespan (e.g. a year or so)
Could we please stop associating open source with security? Don’t get me wrong, I love open source software and it is easier to trust open source software than proprietary, because it is highly unlikely, that they hide stuff like trackers in there. It is also most of the time highly configurable and sometimes even hackable and as a software developer you are able to look into the mechanisms behind the APIs which is sometimes really helpful.
But events like the lzma incident last year and predictable openssl RNG in Debian some time ago (https://lists.debian.org/debian-security-announce/2008/msg00152.html) should tell us, that open source doesn’t mean secure software. And the argument, that there are many people looking at the code is not really true. E.g. many maintainers of the linux kernel only look at specific parts/drivers in it and maybe into some other things they need for that. There are probably only a few people if any (apart from governments), that have read, understood and analyzed the linux kernel in its entirety with all the (open source) drivers built into it and all the possible combinations of configurations. And I don’t want to know how many have done all that for less popular projects. And even if that is done at some point for an upstream project, you would have to check the patches from your distro and if there are any do it all for yourself again. And when the next release arrives you would have to do all that in its entirety again (although with some head start) if a new version arrives (that has, say, at least a thousand lines of code changed, removed or added). And now think about how many big releases come with some software per year. And don’t forget to also include all the dependencies you have to check including the compiler and standard library of the language(s) used.
Of course it is easier to do all that for OSS as an outside party because you don’t have to decompile it, but it is still increadibly hard. And only to be easier to analyze for security risks doesn’t mean to be more secure just like packaging being recyclable doesn’t mean that it will be recycled.