There are lots of cases where this assumption doesn't hold: if you're sampling from a join, or from a table with non-contiguous ID values, for example.
It seems less, not more, likely that this kind of situation would exist in a huge table - what use would such a table be if you could only process it sequentially?
A sufficiently smart database might understand "order by rand() limit n" and do reservoir sampling on row id's behind the scenes. I wouldn't count on it, though.
If you set it up in advance, you could also use reservoir sampling to avoid even storing all the data in the first place, while still saving a random sample.
In fact, i think this is the underlying meme of 'big data' - that it's now plausible and useful to do analysis over all of a large dataset, so you might as well build everything around iterating over the entire table regardless.