Filter Entries by Matrix Block
You'll have to query all matrixes in that field with that type first. With these results, you should then be able to query the entries that are relatedTo these results.
You'll end up with
{% set matrixes = craft.matrixBlocks()
    .field('matrixField')
    .type('video')
    .all() %}

{% set entries = craft.entries()
    .section(['events', 'sessions'])
    .orderBy('datum desc')
    .matrixField(':notempty:')
    .relatedTo(matrixes)
    .limit(30)
    .all() %}

Documentation about relations can be found here