Coming from a Linode employee, yes and no - you're not charged for uploading to an object storage bucket in general, but if you upload from a Linode over IPv4, that counts against your transfer quota for that Linode. Also, any outbound data (including to the same DC) is billable. It's also worth noting that any IPv6 uploads don't count against the quota. Here's Linode's pricing doc:
https://www.linode.com/docs/platform/object-storage/pricing-...