keys with meta-prefix cannot be longer than two chars?

[Artur Malabarba]

The following is quoted from the header comments in isearch.el

;; The key bindings active within isearch-mode are defined below in
;; `isearch-mode-map' which is given bindings close to the default
;; characters of the original isearch.el.  With `isearch-mode',
;; however, you can bind multi-character keys and it should be easier
;; to add new commands.  One bug though: keys with meta-prefix cannot
;; be longer than two chars.

That last sentence is intriguing. I tested with this snippet and it
didn't hold true for me.
    (define-key isearch-mode-map "\M-6lo" (lambda () (interactive)
(message "HI")))

Does anyone know its origin and may I delete it?