This isn't true. RSUs are taxed as income at vest time, so if you're granted 100K in RSUs in 2020, and they vest in 2023, come 2023, if the stock has increased 25% to 125K, you'll be taxed on 125K of income. No capital gains involved anywhere.
If you get RSU's in an American parent company your screwed its just income there's some calculations on the relevant redit r/UKPersonalFinance - basically its treated income and you also don't get the legal protections
The example shown on the site you quote has a 56.53% effective rate - don't forget you pay NI as well
As I said very poor I paid zero tax on my two share saves and my current EMI is at the 10% rate.
Update the 56.5% rate is after you put 20k into pension the actual rate is just under 70%