It certainly was before the Pharaohs and perhaps before the glacial epoch.
Previous to the present period there was what is so well known as the Glacial epoch.
The Glacial Epoch is the subject of the second part of the book.
The answer is, they were stranded there at the end of the Glacial epoch.
Indeed one writer on the subject goes so far as to place their foundation at the beginning of the second Glacial Epoch!
Suppose, for example, a return of the glacial epoch and a spread of polar climatal conditions over the whole globe.
I quite agree with what you say of almost certainty of Glacial epoch having destroyed the Spanish saxifrages, etc., in Ireland.
The curve suggests a glacial epoch culminating about 28,000 years ago.
Try, then, to picture to yourselves the probable aspect of those regions during a glacial epoch.
But putting that consideration aside, what evidence have we that the Isthmus of Panama was submerged during the glacial epoch?