I really don't understand the difference between a "bot" like FL and these web based services.
To me, if you're not using the IG app then you're "botting" or "cheating" in some way.
Someone said it because they are using private API's and it's easily bannable. Also, a lot of bots out there just scrape and use the info for spam or malicious intent. That's why people farm hundreds of IG accounts and build them up liking/unliking, following/unfollowing, auto-commenting non-sense, etc., because by the time they are done they have like 10 left, Lol. (You know what I mean)
There is no service, without for example HootSuite's 'schedule and confirm' method that can provide legitimate scheduling like on Twitter using their API, etc.,
The only one I know of was also already mention, which is
http://schedugr.am. The reason this works and seems pretty safe too is because they have a pool of mobile devices and essentially manually post the photo, caption, and even hashtags that other schedulers currently lack. They do have very strict rules though. But it is about as close to legitimate scheduling that you will find.
The reason this works is again, they are simply just posting for you, and it costs money. But with that said, they seem to have good reviews so far and state they will never disclose to IG people using the service.