I'm amused that you post this viewing it as a vindication of Rowbotham's theories by de Morgan, when to me it appears that he is saying that Rowbotham provides useful intellectual exercises for students of astronomy to question their teachings, but that his theories need not be taken seriously.
Yes, Rowbotham does provide useful intellectual exercises for students of astronomy in learning that the truth that the the earth is flat.
You missed "nor allots them any future inconvenience" bit meaning that Rowbotham did not cause any problems for RET either.
That's funny, I took the last sentence of Morgan's statement to say that Rowbotham does not call his opponents liars, nor does he "allot them any future inconvenience," meaning that he does not resort to insults either.
Here's a similar commendation after a lecture at the Devonport Mechanics' Institute, an institute of higher education:
"PARALLAX AT DEVONPORT.--On Wednesday evening last the gentleman adopting this cognomen, and who has been creating a great deal of interest in this locality during the last few weeks, commenced a series of lectures at the Devonport Mechanics' Institute. The reasoning of 'Parallax,' which he has termed Zetetic, is so astounding and diametrically opposed to the great Newtonian theory which has obtained in the world for hundreds of years, that he has often been ridiculed as a crude experimentalist, abused as a false teacher, and even accused of mendacity. He has borne these harsh expressions and ungentlemanly imputations calmly and patiently; and it is but just to say that, in his lectures, he has always courted the fullest inquiry--stating that his only object is the elucidation of truth, no matter what it may be or what it may lead to; and that in his discussions he is courteous in hearing and candid in expression. That he is a clever man, and that he has studied his subject deeply, there can be no possible doubt; and it is certainly the case, whether he is right or wrong, that his arguments are exceedingly plausible, and that he has much the better of his opponents in discussion. Unfortunately those who have entered into discussion with him have in nine cases out of ten become excited and lost their command, while 'Parallax,' remaining cool and calculating, has thus, apart from his demonstrations, been enabled to gain an advantage over them in reasoning. On this occasion the discussion became very warm, and ungracious imputations were made, which 'Parallax' said resulted from a fear to face the consequences resulting from new and true ideas. The demeanour, respectful bearing, and candour of 'Parallax' bear out his assertions that his object is the elucidation of truth; and he appeals to his audience to disprove his statements, while he undertakes to prove them to be true. He is fair in every way, and it is unjust, nay, it is something worse, to treat with disrespect a lecturer of this character."