I went to Glastonbury (only once) in 2019 with mates and it wasn't the there were no black people there - just very few. Part of this may be because the big acts act Glastonbury (like Download) have traditionally leant towards rock. You're not going to get that many black people because generally that music attracts a largely white crowd.
...compared to Download. Especially Download at Finsbury Park, which featured 'the big three' - Cardi, Nicki and Megan.
Glastonbury has been changing with R&B and rap acts (Janet Jackson, Lizzo, and Janelle Monae were on in the year that I went), but the biggest barrier to black people attending isn't the acts - it's the cost. You can spend an easy £1,000 (tickets, camping equipment, food/drink, transport). What Sir Lenny would also not have seen is very many working class people there either.