Mr. Lee said the sentence was incorrect because the portion I have shown in red should be changed to "이십니다." In other words, he was saying that the ~이었습니다 pattern should not be used in such situations. He explained that since the person who created hangeul could not have changed over time, the 었 in 이었습니다 should be dropped.
한글을 창제하신 분은 세종대왕이시었습니다.
King Sejong was the person who created hangeul.
To give you an idea of what Mr. Lee was trying to say consider the following English sentences.
- Thomas Edison is the man who invented the light bulb.
- Thomas Edison was the man who invented the light bulb.
- Bill is the man who passed out at the party.
- Bill was the man who passed out at the party.
Mr. Lee was essentially saying that among examples 1 and 2, example 1 would be the more correct choice since the act of inventing the light bulb could only be applied to Thomas Edison. However, among examples 3 and 4, example 4 would be the correct choice since Bill would not be the only person capable of passing out at a party.