Load caused by a DDOS attack will be lower if blocked by .htaccess as the unwanted connections will be refused early and not allowed to call your PHP scripts.
Take for example a request made for the login script, your apache server will call the PHP script which will (I'm assuming) do a user lookup in a database of some kind. This is load.
Request <---> Apache <---> PHP <---> MySQL (maybe)
If you block and ip (say 1.2.3.4) your htacess will have an extra line like this:
Deny from 1.2.3.4
And the request will go a little like this:
Request <---> Apache <-x-> [Blocked]
And no PHP script or database calls will happen, this is less load than the previous example.
This also has the added bonus of preventing bruteforce attacks on the login form. You'll have to decide when to add IPs to a blocklist, maybe when they give incorrect credentials 20 times in a minute or continuously over half an hour.
Firewall
It would be better to block the requests using a firewall though, rather than with .htaccess. This way the request never gets to apache, it's a simple action for the server to drop the packet based on a IP address rule.
The line below is a shell command that (when run as root) will add an iptables rule to drop all packets originating from that IP address:
/sbin/iptables -I INPUT -s 1.2.3.4 -j DROP