See my other, more detailed, comment on this thread, but the reason for this is that phonics is part of the solution, but it's not what creates fluent readers.

Most phonics programs do not treat automaticity as the goal, so kids with effortful and slow decoding count as "reading". The science is very clear on what causes this lack of automaticity and what exercises best correct it, but most programs ignore it.

So kids with no deficits will develop mostly fine, but those with them will look to be "reading" but will have trouble once the material requires too much of them.