Do the following in Sigil:
replace it with:
<hr class="sigilChapterBreak" /><h3
Then select Split at Chapter Markers
from the Edit
menu and regenerate the TOC.
BTW, the ePub also contains a couple of syntax errors. Click the green check mark to search for them.
Most readers have limited memory and cannot open ePubs that consist of single large page. It's best to test ePubs with ADE
instead of the FF extension, which is more forgiving.