Pirat I was a bit confused with your description, I understand this is the context ?
#header .container .Header-title
is currently h1, you suggest not using a title tag
.DiscussionPage-discussion .Hero .DiscussionHero-title
is currently h2, you suggest using h1
I'll let our more SEO-savy people comment on this. The current title structure goes back many versions of Flarum, it might have never been changed since the beginning.