Hi birdhouse,
If you are buying individual layers and components such as one or more latex layers and a cover and putting together your own mattress then I would consider it to be a DIY mattress yes (in other words … you aren’t buying a mattress that is being sold “as a mattress” … only as individual layers and components).
If you are also purchasing a suitable cover and if you are confident that the latex core and the cover would be a good match for you in terms of PPP (Posture and alignment, Pressure relief, and your personal preferences) and that you will sleep well on it and it also compares well to the other options that are available to you either locally or in the online lists that are linked in the tutorial (in the optional online step) based on all the other parts of your personal value equation that are important to you then it would certainly be worth considering.
Phoenix