Plagiarism is obviously a word with very strong negative connotations. If you want to discuss the technology and it's differences between a different solution that tries to solve the same problem and not accuse someone of stealing, it's usually best not to use this type of language in general.
That's because Mastodon doesn't have direct messages. It is not a chat platform. You can bend the privacy settings to publish posts similarly to DMs, but no one should use it as such.