1. Typing lag provides something to do while you're lagging.
2. It's very possible for a host to not notice that the server is lagging, and may need to cut down their server population or close that random program that decided to start auto-updating at the worst time.
3. If it's client sided, it's also good to know. Sometimes with slower internet connections for clients they may not be able to handle a high population server, or the pesky auto-updater situation again.
On more of a side note:
4. Typing lag has virtually no effect on lagging the server. Yes, it does send information over the tubes, but it's very small compared to all the different things the server is updating several times a second. If the server is lagging in a single hard spike, then the messages simply wait till they can make it through. If it's more constant lag, the server doesn't have enough bandwidth to push everything through and each message is a one time drop of a connection in a pipe that is too small.
5. Single people are rarely the cause of overall lag in a game. I'm not sure that it is even possible (other than host of course) for single people to cause everyone to lag with Blockland's engine. I know that back (a long time back) when I had dial-up I could be on a server packed with people and be lagging like crazy, but everyone else was unharmed. Most likely people are just trying to find something to blame other than admitting that the server needs better bandwidth to host the number of players it has, that is unless,
6. Spam (mainly projectile or vehicle) which is lagging the server. Bringing the host's attention to this as soon as possible (which they may not notice right away) is very important for the smooth operation of a server.
/wall of text answering your question about why people say it