What are you trying to do with the mentions? Mention tags are represented in ActivityPub as a separate tags object, like this:
"content": "<p><span class=\"h-card\" translate=\"no\"><a href=\"https://mastodon.social/@photomatt\" class=\"u-url mention\">@<span>photomatt</span></a></span> 👋</p>",
"attachment": [
],
"tag": [
{
"type": "Mention",
"href": "https://mastodon.social/users/photomatt",
"name": "@photomatt"
}
],
For example, if you’re trying to figure out if a blog is mentioned, you look up the id of the blog in the tag array to see if a Mention tag exists for that blog.
It seems like you know this already, so I’m not sure I understand what you’re looking to accomplish.