It does not say that you have to resize the filesystem. It does have a section describing how to do it. Notice the page is not structured step 1, step 2, step 3. It is simply organized under various headings.
If you did want to to resize, the process referred to there is fairly standard but the way it is described is very abbreviated and might easily confuse the unfamiliar. The two basic steps are:
Change the partition table entry.
Restructure the filesystem.
These two steps correspond to the distinction between a block device partition and a filesytem. The former contains the latter. Physical storage devices, as I am sure you are aware, can be "partitioned" into a number of discrete segments. Each one of these is referred to as a partition.
Most often to be made useful a partition is then formatted to contain a specific kind of filesystem, but a filesystem and a partition are not synonymous and the distinction is very clearly defined although it may often be ambiguous to many users. In any case, the point in step one about deleting the partition really just means just deleting the entry in the device's partition table. This does not affect any data in the filesystem on that partition. The idea is to then re-create the partition entry and make it bigger. This is necessary before you can make the filesystem inside it bigger. It is kind of a hands on method but remember -- the pi is all about learning.
There is a very important caveat here that the ubuntu mate crew neglect to mention, namely, when you re-create the new partition, it absolutely must have the same starting sector as the one you just deleted, or you may end up destroying or losing the filesystem inside. There is the assumption on that page that using the
fdisk defaults will handle this. I cannot say one way or another WRT the ubuntu mate image (it is possible this is true), but I can tell you for certain this is NOT true using a Raspbian image, i.e., if you followed those directions, you would be making a mistake.
Perhaps they ran out of space after what was required at the top for all their "Please send me money, I created an SD card image" stuff (excuse my sarcasm and do not let that stop you from donating, I am sure they mostly do a good job). The pi may be all about learning but beware that does not always mean it is always about teaching, so sometimes "learning" might mean "the hard way".