There is a very fundamental Wittgenstein-ian problem here; words are only given meaning by the objects/feelings/ideas which they describe. We're using the term "consciousness", but what exactly does that mean? First of all, there is not a universally accepted definition of "consciousness", and even if there were, I'm not sure that it would be one that is useful for humans to reason about, given the popular definitions that are fluttering around today.
One of the most popular definitions is something along the lines of "consciousness is defined as the experience of what it is like to have sensation, emotion, thoughts, etc...". Ok, now what are sensations? Emotions? Thoughts? Oftentimes people will say "they are all parts of the experience of consciousness", and now we have a meaningless loop of a definition, akin to "a guitar is an instrument, and an instrument is a more general case of a guitar", except we have more observable information about guitars and instruments, but do not have much of a clue to reason about "consciousness". As Wittgenstein said in his Tractatus, "whereof one cannot speak, thereof one must be silent", meaning that if we want to ascertain some higher truth about some subject, we must reduce it to logic. We have an idea of the sort of thing that the term "consciousness" refers to, but we don't know how to approach it analytically, therefore we cannot make any reasonable conclusions about it (yet).
For the sake of discussion, let's not worry so much about what's been said in the previous two paragraphs. This isn't too bad to do, because the argument I'm going to make is suitable for the fuzzy definition of "consciousness" that I'd mentioned at the start of the second paragraph.
So, what the hell does the person who wrote that article mean by "You may know beyond a doubt that you exist, but your ‘I’ could still be an illusion“? My understanding is that they're referencing something similar to "The Matrix" (movie), where one thinks one's sensations and perceptions occur in a spatiotemporal world (for a logically sound, but controversial, definition of "world", see again Wittgenstein's Tractatus), a world which is the boundary of all things that could possibly exist; yet one day one is shown that this world is just a simulation, our world W is just a subworld of another world W1, which may be a subworld of another world W2, etc. My interpretation, in summary, is that what we perceive to be "absolute reality" is maybe not so absolute after all.
The paragraph above was a more metaphysical interpretation. For a more existential interpretation that deals directly with sensation and perception, look up "brain in a vat thought experiment". It is the existential analogue of the metaphysical interpretation above.
A quick synopsis, a tl;dr if you will: We need a well-defined, universally accepted notion to which this term "consciousness" refers, and shouldn't make universally accepted conclusions based on anything else. Ignoring this, the "illusion" we're describing is that our supposed "objective reality" isn't necessarily all there is, and our world may be part of a higher world, which is part of an even higher world, etc.