Is it a requirement to use an SDK to develop programs/apps for Jolla?

SDK is really a really big no no in general. Does Jolla not have the tools to install for example GCC?

One of the worst things about Nokia N900 is the lack of libc and core GNU components.

As far as I know Mer is GNU and not busybox, right?

Does Jolla support libc/make/gcc?

If not, then Jolla is no better than N9 which was pretty bad compared to N900 which again was not perfect. The natural way forward from N900 which used Busybox was ofcourse full GNU and replace GTK with QT and keep everything else.

This unfortunately did not happen, which still makes Nokia N900 king of mobile devices in 2015, which is quite sad considering it was released in 2008.

The mobile marked sucks!