Sorry but you need to read the whole sentence/paragraph, not just cherry-pick a few words that you think support your case. And if you think the text is ambiguous, it would be incumbent on you to acquaint yourself with the relevant statutory provisions because, from a UK tax law and tax penalty perspective, if you ended up failing to notify HMRC of your source of income, and/or failed to properly return and self-assess your tax liability arising from that source, claiming "ignorance of the law" is not a valid excuse.
The paragraph you have lifted those words from is aimed at tenants in cases where you, as landlord, do not employ an agent to manage the property for you. The paragraph reads:
"If the landlord does not have letting agent, you will need to operate the NRLS. If you’re a tenant who pays rent of £100 a week or less, you do not have to use the Scheme unless you’re told to do so by HMRC"
So, unless your tenant pays less than £400 per month rent, they must operate the scheme and withhold tax at 20% from the rent they are paying you. They must then account for that tax to HMRC.
If they are not required to operate the scheme, you still remain liable to self-assess your UK income, albeit having been received gross, rather than being received net of basic rate tax, which would be given as a credit in your tax calculation. In all circumstances, you remain liable to return the UK rental income, calculated in line with German tax law, for the purposes of setting the rate at which you pay tax on your income that is taxable in Germany.