From Queenpedia.com

At some point in Queen's very early days reviews of either some concerts or their first BBC sessions which praised their "synthesizer playing" despite the band's distaste for such equipment at that time.

As a result, the line was inserted onto the first album to indicate that all sounds were made by "real" instruments (although there was also tape trickery - backwards, sped up, slowed down, etc etc throughout the recording history).

Later albums featured similar advice, up to The Game by which time synthesizers had, in the band's opinion, improved to the stage where their use wouldn't compromise the sound of their songs. Later albums featured synths as much as any other instrument.