Sorry badly worded on my part. They can use other platforms alongside steam. I really meant they cannot use them instead of steam without losing a massive audience. And likely making their game unprofitable. At least if they are not a very large game studio already.
And then if steam is pushing restrictions on what they can do on other stores then that is edging into the relms of abusing their monopoly.
You can choose to buy new things on other platforms. But you lose access to everything you bought on steam if you choose not to use them anymore. That is a form of locking to their platform. You cannot just move you library to another platform.