Thanks, that does work. Confusingly, the key file is still in place at the old url, so that it gives an expired certificate error rather than a file not found error. Now, if only the webmasters at the matrix and hackster sites, all of which give the erroneous link, knew of this important change. Or if the “dev team”, to which corporate headquarters directed my email inquiries, knew about it.
For anyone else needing it, here is the correct sequence of commands to load the matrix kernel and software onto the pi and prepare the matrix for re-flashing: