How to calculate the number of hours left?
You can use the date filter to format the DateInterval to your needs, I guess you
are looking for the total hours until the event, so you have to do some math.

Because there's still some strange things going on with Craft DateTime objects, do what I
recommended in the question you mentioned, convert it to a string in a common format that includes time and timezone information
and then use Twig's date function to convert it to a Twig DateTime object.

To get a "DateTime string" in ISO8601 or UNIX format use the iso8601 or getTimestamp method or the date filter with the corresponding parameter.

{% set dateTimeEvent = date(inschrijven.datumEvenement|date('c')) %}
{% set dateTimeNow = date(now.iso8601) %}

{% set dateInterval = dateTimeNow.diff(dateTimeEvent) %}

{% set days = dateInterval|date('%a') %}
{% set hours = dateInterval|date('%h') %}
{% set totalHours = days * 24 + hours %}

{% set upcoming = dateInterval|date('%R') == '+' ? true %}

{% if upcoming %}
    Event start in {{ totalHours }} hours.
{% endif %}