HSTS includes subdomains
The HSTS header includes includeSubDomains, so the HTTPS-only rule covers hosts under this name.
Why it matters
Subdomains are a common place to serve a forgotten HTTP endpoint that would otherwise bypass HSTS on the parent.
What to do
Keep the directive only if every subdomain can answer on HTTPS.
Reference
HSTS max-age and directive syntax are defined by RFC 6797.