Ask HN: How do I convince client to not use Flutter for landing page?

I have this one client that insists on using flutter for their landing page. This is from the fear of not understanding React by their own dev team, as I recommended Nextjs for performance and SEO. I told them that the middleground to this problem is just plain HTML and CSS, but they're keep insisting on using Flutter. Flutter's website (https://flutter.dev) don't even use flutter for their website, it's ridiculous. I wrote this list of reasons not using Flutter for their landing page:

1. Bad performance 2. Bad SEO 3. Limited libraries for extended features like animations, etc 4. no other upside compared to other frontend js frameworks, besides they already have the flutter dev team

What are the things that I can propose to convince them? Thanks