snart : soon, nearly : the contrary of in a long time - She is snart 23 year old, (not yet but almost) - It is snart morgen : It is nearly the morning ( in an hour) - You are snart at work : you will arrive soon at work ( Even if you are late for work, you will be there soon)
tidligt : early, the contrary of late. - tidligt fødte børn : Child born prematurely - tidlig morgen : it is early in the morning - tidlig at work : You are in advance for work
In the sentence above I understand something like "it is not to soon to do something ( to go fishing), too early in the morning"
"It is too late" would be "Det er for sent", which is more common to say, and "Det er ikke for tidligt" would only be used as the answer to a question such as "Er det for tidligt?" (Is it too early?) So it can't really be used as meaning "It is too late" Also, like in English "Not a moment too soon" the Danish translation "Ikke et øjeblik for tidligt!" contains the same hint of sarcasm.