Hello again.
Ok. It's fine for you to be sceptical of these claimes. But, maybe we should withold from describing the output of highly-trained professionals as "speculation" until we understand it to a level where we are qualified to make that judgement. Isn't that reasonable?
For instance, when Richard Dawkins wrote off the entire field of theology in a similar way, people who understand the field suggested he was mistaken. A reasonable person would admit that they don't know the field and aren't in a position to dismiss claims made by the pros.
Don't you think so?
Say I handed you a special book and each time you opened the book to read it the text changed a little (on its own and at random). You might not ever see the book change from a treatise on steam energy into a play by Shakespeare in "real time". That wouldn't rule out the possibility that you could have a book that reads as a physics text later read as a tragic play at some time.
So, by analogy, we shouldn't rule out the possibility that a lineage that produces fishes couldn't later produce mammals. We don't expect to see fish give birth to mammals but if we look at the fossil record we can infer that some lineage that produced lobe-finned fish about 400 million years ago gave rise to all the wide variety of tetrapods that we see now, including mammals.
I'm doing my honest best. Take care.