An interesting thread. Suggest you read the book, and also Shakespeare's "all that glisters is not gold" (which Tolkien is quite explicitly playing upon). This poem refers to Aragorn, who is not just a normal man - in fact, on first meeting him, he appears to be deeply suspicious, the kind of character one should steer well clear of, and goes by the ambiguous name of Strider - but despite his unpromising appearance he turns out to be (plot-spoiler warning!) the Heir of Isildur and the True King-that-Returns, Dúnadan.
Whilst the construction of the first line may not appear at first glance to be "logical" or even strictly grammatical, both Tolkien's "All that is gold does not glitter" and Shakespeare's "All that glisters is not gold" employ poetic/dramatic licence - in many a poem is the order of words apparently illogical, but the words lure the mind into finding the truth behind the ambiguity. There is a double meaning here:
(i) Things that glister or glitter may turn out not be valuable at all - i.e. "not all that glisters is gold", in other words the glint of bling may turn out to be false or valueless;
(ii) Some things of immeasurable worth may not only NOT glister but appear to be dull and unattractive, or even apparently repellent, i.e. "Not all that is gold glisters". This is similar to the adage that "old money whispers, new money shouts" - true value may not be obvious, indeed it may be hidden, unlike the false which may sparkle and be brash but turn out to be cheap.
Tolkien draws on some of the oldest myths and legends in the world - beware the lure of red gold, it may not be as valuable as it appears; and sometimes the field of diamonds lies hidden beneath apparently the most ordinary, or even ugly, of coverings. Enjoy the quest.