"Un deg pump"
Yes, and to cinq, cinco, quinque, pente and to English five, for that matter -- they are all descended from a common Proto-Indo-European word which looked something like *penkwe.
Welsh is in the "P-Celtic" group of languages which turned earlier qu-/kw- into p-, hence the relationship between pump and Latin quinque: at an earlier stage within Celtic, the word was something like *kwenkwe as well.
If you go far enough back in the language trees, perhaps it is, but not in the last couple of thousand years. There is a lot of research going on in the field of historical linguistics, tying it in with recent work in genetics and archaeology, so many of the ideas about the timings and routes of the origins of the Celtic languages are being constantly challenged. One multi-disciplinary set of research papers has been bound as a volume, for example - 'Celtic from the West' by Cunliffe and Koch (eds) - heavy reading but very interesting.
As explained in the course notes, Welsh has two counting systems. The old one is based on twenties ('vigesimal'), rather like French. The other is the simpler decimal system, which is much more widely used nowadays. I think that the course notes have a link to an external site which shows the two systems side by side.
The vigesimal system is taught on this course for dates only, for example yr unfed ar ddeg ar hugain o fis Mai (the thirty-first of May). You may also sometimes still meet it being used for people's ages and for money.
Pymtheg is the older way of saying un deg pump, although some people do still use it for general counting because it is so short. The problem is that it can be easily mistaken for pum deg (hanner cant in the old system). Similarly, the older deuddeg (12) is easily misheard for the newer dau ddeg (20 - ugain).
This is explained in the notes for this section - you need to go to the web-site version to see these as Duo does not yet make them available on the app.
pump, chwech and cant all lose their last letter when used before nouns. As explained in the notes, the course material does not always do this, an error which will be corrected in the future.
- pump, chwech, cant - five, six, a hundred
- pum afal, chwe oren, can car - five apples, six oranges, a hundred cars