Firstly, there's a certain very-1980s racial bias in putting forth the idea that an anonymous white man, rather than someone like the actual African-American Chuck Berry, actually invented rock'n'roll (which obviously is inherent both in that scene in Back To The Future and in a similar scene in the Australian 1980s film Young Einstein). After all, by July 1985, when the film was released to cinemas, rock music was clearly mostly quite white - Springsteen and U2, the Rolling Stones and Bob Seger - so it made sense that it was really invented by a white person...didn't it? Ahem.

Secondly, in terms of the actual timeline of this reality, by November 1955, what was clearly recognisable to future audiences as rock & roll already obviously existed. If you look at what's actually the first Billboard Hot 100 chart which, incidentally, was published in the very Back To The Future-adjacent November 12th, 1955 edition of the magazine, you'll see that rock & roll is not absent: Chuck Berry's debut single Maybellene was #41 in the charts that week (and, as a song released in July, was pretty much on the way out by November). Bill Haley and the Comets's Rock Around The Clock had been one of the biggest songs of the year (topping sales charts in July), and in November was still at #56. Fats Domino's Ain't That A Shame was at #67. Later on in that issue of Billboard, a writer talks about how 1955 was the year that R&B really infiltrated mainstream pop in a big way (ironically, 1956 would be even bigger for R&B). An African-American R&B band playing the prom night of a Californian high school in a white area might not have happened the year before.

So 'Johnny B. Goode' definitely would not have been like nothing you'd ever heard, if you were that journeyman R&B guy playing in Chuck Berry's cousin's band. Almost certainly that R&B band would have been playing 'Maybellene' and 'Rock Around The Clock' that night - they were major R&B crossover hits at the time that the crowd presumably would have wanted to hear. Indeed, that R&B band, as they were playing along to 'Johnny B. Goode', probably would have been thinking "eh, it's a bit 'Rocket 88'" (the 1951 Jackie Brenston & His Delta Cats track that was obviously an influence on Berry, and sometimes gets called the first rock & roll track). If Chuck Berry's cousin called him in November 1955 and said 'you know that new sound you're looking for? listen to this!', I can only imagine Chuck Berry would have thought 'ah, who's ripping me off this week?' (after all, Marty Robbins' cover of 'Maybellene' was by November 1955 a hit on the country charts).

There are distinctions between rock & roll and the R&B of the early-to-mid 1950s - some of the feel of 1950s rock & roll is derived from country music, and 'Maybellene' is somewhat unusual amongst R&B of the era for having a noticeable country music influence (it's often considered to be more or less an adaptation of an old country tune, 'Ida Red'). Nonetheless, the singer-electric guitarist Sister Rosetta Tharpe once famously said, 'Oh, these kids and rock and roll — that is just sped up rhythm and blues', and she's not wrong. The early rock & roll tracks like 'Maybellene' wouldn't have shocked anybody who had been listening to early-to-mid 1950s R&B - they were just a bit more uptempo than what a Muddy Waters or B.B. King were doing at the time.

The 'twelve bar blues' chord progression used in 'Johnny B. Goode' (the thing that McFly refers to as a 'blues riff in B', which has a chord progression something like I I I I IV IV I I V V I I) and variants on it were a fairly common presence within popular music more generally around the middle of the 20th century. It's present in the verses of a big band swing track like Glenn Miller's 'In The Mood', it's present in a jump blues track like 'Choo Choo Ch'Boogie' by Louis Jordan, it's present in a cool jazz track like 'All Blues' by Miles Davis. You better believe that musical styles like the Chicago blues (the house specialty of Chess Records, Chuck Berry's record label, which is often seen as electrified Delta blues, played by bluesmen from the Delta who ended up in Chicago after the Great Migration playing electric instruments) used twelve bar blues chord progressions all the time. Chuck Berry's cousin's R&B band likely knew exactly where the changes were going to be.