The architecture of the Internet can only offer a 'best effort' class of service so it's impossible for ISPs to guarantee speeds. At best they can offer an SLA to give the customer concessions (free/discounted service) if they fail to meet the terms of the SLA.
That's true. But in this case the ISP isn't even making a good-faith effort to provide X Mbps worth of transport across their own network.
I get that once the traffic hits the broader 'net that all bets are off. That's true and nothing can be done about it.
But in this case the ISPs aren't even making a "best effort" on their own network. They're deliberately not upgrading congested links where their peering counterparties are willing to upgrade. That is not a "best effort" class of service.
