>> Hmm... no, AFAIU, it means that (by default) you cannot replace the 
>> commit history of a public repository with commits which do not have 
>> the HEAD of the public repository as their parent.  E.g. if your public 
>> repository is
>> 
>> A - B - C - D
>>              ^ HEAD
>> 
>> you cannot change it into
>>
>>      B - C - D
>>    /
>> A - E - F - G
>>              ^ HEAD
>
> You can even do that (if 'force pushes' are not disabled in the 
> repository's settings).
>

Yes, I said "by default": force pushes are disabled in the default 
settings of Git repositories, unless they have been enabled you cannot do 
that.