I mean if we are going to talk about vendor lock-in, Epic exclusivity is literal vendor lock-in.

Insert Princess Bride meme.

The vendor doing the lock in is literally the key aspect in vendor lock-in, hence why it's called VENDOR LOCK-IN. One of the defining principles of vendor lock-in is making their proprietary tools difficult to disentangle from or having restrictive contracts for their platform.

First off you're not even required to use or integrate Steamworks to release on Steam, so no vendor lock-in there.

Having the lack of foresight or have skill issues or budget to initially move to another platform and using their version of achievements and then simply hardcoding all achievements, login checks, or whatever else you want to use with direct calls of the Steamframe works calls within game's core algorithms instead of taking time to properly isolate and abstracting those calls outside of your game's core code in my opinion doesn't equate to vendor lock-in, especially when again, you are not required to integrate Steamworks to begin with if you so choose and still release on Steam.

Like you said, Epic has their own propriety API for their launcher and backend calls, so does GoG but just having that doesn't make them guilty of vendor lock-in, you have to also make it prohibitively hard or expensive to move to another platform, and again not having the wherewithal to abstract API calls isn't making it prohibitive hard or expensive but you know what is? Epic exclusivity.