Your NCT group sound like an unusually driven bunch of parents! I have three children, and I've barely known anybody who was potty training at 20 months old. I assume that what's happened is someone's had a copy of the book "Oh Crap" and it's been passed around the group. There are some useful tips in it, but 95% of the book is utter horseshit honestly and I expect most people to give up and try again in about 6/9 months. Maybe 1-2 will get lucky and find that it works well for them but it's really luck at this age if it does.
The thing I don't like about the book is that the author spouts a load of confident rubbish in it, and one of her main things she repeats a lot is this magic age of 20-30 months, which she considers to be the peak readiness window for potty training, and if you miss this it's all over and you will have a terribly awful time. I just think she's wrong. IME she is totally wrong. All my children trained somewhere between their 3rd and 4th birthdays, and they seemed to get the hang of it a lot easier than any time we tried when they were younger. Now, I know that this is something where the current fashion is changing - up until fairly recently, the message we kept getting from all angles was "Wait until they are ready!" and now there are official bodies going ah, actually, no, sorry that was wrong, we have loads of children starting school not properly potty trained, you don't need to wait until they want to do it, you should encourage them to try from about the age of 2/2.5. (Which I do agree with).
Another thing the author keeps banging on about in the book is that when a child can recite the alphabet, that means they are ready to potty train, which could possibly be why some of them are trying to encourage this? I remember finding it such a bizarre claim when I read this - she actually refers to the alphabet song specifically, which I think must just be a lot more prevalent at an earlier age in the US where she is from. In the UK, I just don't think this song is given so much importance or seen as a major nursery rhyme - it might be something that they sing at nursery or school age, but even there it's not that popular because most schools teach phonics and so learning the letter names is a bit confusing, not to mention the whole thing where young children sing "ellemenopeeee" not realising that it is actually L, M, N, O, P.
Anyway, sorry, this has turned into a bit of a rant - as others have said though, speech development is highly variable at this age, and if he's at less than 50 words (which is fine at 20 months) then he's not yet at the stage where they are interested in rote-learning stuff. That tends to come IME more when they have the explosion from a few dozen words to a few hundred - they become word sponges and that's when they like to copy longer strings, like nursery rhymes and if you want you can teach them to name all sorts on command like colours, shapes, numbers, body parts etc. It sounds like some of the DC in your NCT group have had this word explosion, and your DS hasn't yet, but both are normal. With the rote learning, it's all fine and nothing wrong with it, but it's not necessary and particularly for numbers and letters, it doesn't teach them anything about what the numbers or letters mean. Personally I found it more fun and natural to teach numbers incidentally, by doing things like counting steps, counting bits of food or toys to share, counting blocks to make a tower the same, counting 123 go etc.
Remember your LO is still very little - they are only one and a half! Don't worry too much about things like structured activities and classes. They are amazing little scientists at this age and they are fantastic at exploring and learning by themselves. Two things I would absolutely recommend are looking up play schemas, which is a way of understanding toddlers' play which seems random or sometimes destructive, but is actually about understanding the world. And then look at something which explains how they learn socially - I loved The Continuum Concept although it is a bit mad, outdated and hippyish - a more up to date version is probably Hunt, Gather, Parent. This is also fun. https://www.ted.com/playlists/289/the_genius_of_babies