Yes I've dealt with this issue (to be more precise multi-cloud rather than migration, also used other TF providers in tandem like the Kubernetes one), and yes your TF resources are provider-specific, but being able to handle it all with the same tool instead of having to deal with an awful vendor-specific tool (to be charitable, because the reality is having to deal with mutiple ones and having them inter-operate through bash glue) vastly helps in reducing and controlling lock-in, and I would argue it's the baseline step you have to take if you don't want to risk your whole business on a single cloud provider not going to the gutter either because they've grown too big for their own good or because they're being squashed.
CloudFormation means jumping into the pit of lock-in and you can only climb back out by digging your fingers into the dirt wall, Terraform means you have climbing gear to rappel down and if you have to, get back up without as much hardship. Sure you have to put the gear on but your descent is controlled and then you can climb back up at your own leisure.
If I get to make the call and I care one bit about the business beyond next quarter, I would always have a clear way out to heavily reduce risks to a situation that we have seen play out many times before with the Oracles and IBMs of the world.
You can’t imagine the red tape moving from Workday and their homespun EMR (Healthcare) written 20 years ago using Powerbuilder.
Also if you are at any type of scale, your IAAC choice is the least of your problems.
Have you actually done a realistic project plan and estimated how much it would cost to migrate infrastructure from one provider to another once you reach any scale - including regression testing, auditing, training, etc? Do you really just use your cloud provider to host a bunch of VMs? Even then it’s not that easy at scale.