Get the fields from a reference entity (content type) in a Paragraph field
The referenced entity is not built yet, so you can't access the formatted field content.

You can get the field value from the database by using the paragraph object:

{{ paragraph.field_testimonial.entity.field_testimonial_titel.value }}


This is unformatted, which might work for a title field.

If you want a formatted output better use a custom view mode for the referenced entity to configure the fields, or in this case one field, if this is all you want to display.