thanks Teppo, worked great after 1 amendment:
on first attempt I got an error:
"Error: Exception: Can't save field from page 1028: /blog/posts/test-post/: Call $page->setOutputFormatting(false) before getting/setting values that will be modified and saved."
so, I did as the error suggested end ended up with:
$comment = new Comment;
$comment->text = "my text";
$comment->cite = "my name";
// etc.
$p = $pages->get('/about/');
$p->setOutputFormatting(false); // added
$p->comments->add($comment);
$p->save('comments');
checking the PW cheatsheet for $page->setOutputFormatting() makes it clear why that's needed.
applab's post in Is It Possible To Add A 'comment' Programmatically? was marked as the answer