Capital strike is a strike by capital. There can be typical cases, but fundamentally that's what this is - though under platform capitalism, I think platform residents hold a much more precarious position than traditional capital because they answer to more than just the government and the market - the platform forms a second government for them.
A cartel is a group of businesses who collude to take market power as an oligopoly. That's not what this is, because these sellers are striking for platform changes, not consumer domination. You can keep saying it's cartel behaviour, but you're working with a different definition of cartel to the mainstream one. They're colluding yes, but they are not warping market forces the way a business cartel does (the typical example being the lightbulb cartel: https://en.wikipedia.org/wiki/Phoebus_cartel).