Substantial and substantially are sometimes used in overestimations of supporting evidence for one’s thesis, idea, etc.
“A substantial number of scholars have suggested that . . .” or “a substantial amount of evidence lends to . . .” or “substantial attention has been paid to . . .”.
In a context in which “substantial” is unpacked, i.e., the writer gives reason as to the importance of whatever-it-is, it’s use is acceptable. But one should be careful not to use it without such context.
I can imagine, too, that being very familiar with one’s own thesis can lead to such vague use of these sorts of adjectives (or adverbs) because one easily assumes a context for the word’s use without ever putting that context in writing.