Yeah, this EIP specifies an unconditional balance increase with no source, just like how the coinbase reward works.
While it could be nice from an accounting standpoint to try to match withdrawals to deposits in the deposit contract, we would run into an issue as we (likely) end up with more ETH on the beacon chain than started in the deposit contract (due to validation rewards). So you could end up with some kind of negative balance in the deposit contract in the limit. I’d also push back against addressing the deposit contract balance in this EIP to keep the scope smaller.
I know @axic has thought some about the accounting here and he may have more to say.