Let me try to explain what we're talking about. Usually, the domain registrar (strato-hosting in this case) has some nameservers available that work with your account. If you have hosting elsewhere (000webhost in this case), they might also have nameservers to work with that account. Every nameserver can contain several types of records (A, Cname, MX are the most common). Now, to function properly when the possibility of multiple nameservers are involved - one has to be 'the boss'.
When you register a domain name with a registrar, their systems sets up their own nameservers to be 'the boss'. This (or these) nameservers are responsible to contain the various individual records that will connect a hostname to an IP address. If you host elsewhere, this might not be the most convienient setup. What these 000webhost instructions are telling you to do, is to change 'the boss' nameservers to 000webhost instead of your registrars. For simple setups, this will be very convenient because several of functions in the 000webhost cPanel create these individual records for you automatically.
Hopefully that helps with the explanation of what they do. Now, with that said, you don't neccessarily need to have the 000webhost nameservers be the boss. You can manually create the corresponding records at strato-hosting's name servers and it can still work fine. It just takes more effort and careful thought.
I quickly looked through the FAQ's at strato-host and did not see anything that obviously allows you to set your own name servers. Some registrars/hosts do not allow users this permission. If you look at thier FAQ article 2082, it seems that you can for subdomains only.
So, to host at 000webhost, you'll need to create your records manually at strato-host nameservers. At minium, you'll need:
- an A record for @ or * (depending on how their interface works) which points to the IP address of your server at 000webhost. This can be found on the right side of your 000webhost cPanel
- a Cname record for www which points to your @ or *
There may be many other records that you'll need or want, but that should get you started.