oh well. speak about discoverability. many thanks for this hint.
OTOH, I can't think of many big-name mobile apps that don't have some issues with less-than-obvious long-press functionality. (Snapchat seems to literally do this shit on purpose, because it's "fun" apparently.)
IMO that's a general problem with mobile UX... long-press actions have zero affordance. There's no way to tell short of randomly long-pressing on stuff what's going to have an action associated with it.