I've also negotiated with AWS, and both your position and their position strikes me as equally true.
There's certain products of theirs that they just aren't going to negotiate on, because they know they've got you, whereas others the clouds part and discounts rain down.
It certainly used to be this way when AWS had less competition, these days there's an Azure/Oracle Cloud/Rackspace/Google/etc alternative to most of their greatest hits, which gives a greater negotiating edge.
Lambda certainly has more alternatives than Dynamo for example. But I guess the true lock-in is the integration. If you use Lambda, chances are you'll end up choosing S3 and SQS and Dynamo and API Gateway etc.
There's certain products of theirs that they just aren't going to negotiate on, because they know they've got you, whereas others the clouds part and discounts rain down.
It certainly used to be this way when AWS had less competition, these days there's an Azure/Oracle Cloud/Rackspace/Google/etc alternative to most of their greatest hits, which gives a greater negotiating edge.