Sorry but it isn’t kinda immutable because ssh is disabled. A malicious website can install a keylogger and catch your sudo password for example. A customer got some cryptominer running with no ssh enabled.
ssh can be used safely, and heavily secured.
I apologise for my semi-optimal build of sentences. The phrase ‘kinda’ shall indicate that it is not 100% immutable since e.g. software gets installed by the user. The line in question was supposed to include the aspects of the paragraphs above like App installation never on root. 
And you are absolutely correct in advising that being only itself is already a risk. 
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I get what you mean, and to be fair: I have two laptops where the flatpaks are installed the --user, yet my main machine has a testing account “beheerder” which holds no essential data, and of course a “knurpht” account. On this machine I install the flatpaks systemwide. So far did not run into issues.
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