I was using the word “naturalist” in its wider application, including amateur same. I get the impression (lack the time right now, to explore further) that the authors of the book concerned are indeed not professionals in the biology / natural history field, with all that that entails – it would seem rather, that they earn their livings as writers / journalists, and can thus better afford to entertain eccentric / heretical notions natural-history-wise, than if they had academic careers in the scientific community.
It appears to me from the book that these two are, in their natural history interest, mostly on the mainstream scientific page (and widely knowledgeable about same); but are willing to leave the door open a crack – or perhaps a bit more – to other “takes”.
Any such willingness is indeed, as you observe, professional suicide if biological science is one’s career. This overall issue is one which at times gets quite rancorous, in the area of interest in and enquiry into, the matter of “Bigfoot” in North America. I tend to feel that mainstream science’s reaction re the paranormal is understandable, but none too admirable. Scientists ridicule the excesses of “religious establishments” in other times and places, in the matter of attempted enforcing of orthodox views, and stifling of speculation. They consider themselves far more enlightened – I sometimes wonder, with how much justification? Burning people at the stake is not an option these days; but I see at times, surprisingly much, same stuff, within the parameters of what is nowadays allowed, re “getting heavy with perceived heresy”. Being hypocritical, without realising it?