This is exactly right.
One of the first refs I found to the synonymy issue is in Ellwanger. Don't know whether this is derived from earlier work of others, or whether it originated with Ellwanger:
“Ellwanger [1893] - Synonymous, or too-much-alike roses. A drawback to the purchase of new varieties is the knowledge, gained from past experience, that a large number of those sent out as new sorts are not sufficiently distinct from known varieties to prove of any value.…Before enlarging our already cumbersome list of varieties, we think it of great importance to thoroughly sift the sorts now commonly grown, and where two or more varieties bear a strong resemblance to each other in the appearance of the flowers, to reject the inferior kinds. As roses which are synonymous, or too much alike, as regards the form and colour of the flowers, we note the following kinds:…
Many of these roses are identical in all respects save name; the others are certainly too much alike to be grown, even in the largest collections; for though there may exist some considerable difference in the habit of growth of a few of those coupled together, the distinction between the flowers is exceedingly slight, such as can be observed by experts only. I have in every case placed first the variety which seems on the whole., the most worthy of being retained; in a few instances I have found it difficult to make a decision, this is where an asterisk (*) is prefixed to the name. In all these cases (*) we shall make further study of the slight differences which exist between the varieties so as to determine the best; we hope to have the aid of others in this matter.”
Ellwanger doesn't suggest that the roses are necessarily identical, just that the flowers are difficult to tell apart – he makes the point that some are very dissimilar in habit. But a few years down the track it's been taken a bit further and is now part of the NRS rule book. – This from William Robinson's The Flower Garden 1903 edition:
"Synonyms of garden roses. - The following roses bracketed together have been regarded as synonymous, according to the rule adopted by the National Rose Society; and the name standing first in each case, being believed to be the original one, is considered as the true name of the rose, and the one that should stand."

In 1909 Sanders has this to say:
“Synonymous Roses – Following are the varieties of roses which are known by more names than one. On the left hand side we give the correct name, and on the right the synonyms. An intending exhibitor should be careful to read this list before staging his blooms, as should he, for example, stage Mrs Harkness and Paul’s Early Blush [included in list as synonymous] in a stand of six or a dozen distinct varieties, he would be disqualified.”