If the Reddit post is genuine, it was a transwomen, aka a biological male expressing their gender as female.
According to possibly them and aligned with media reporting they approached a teen girl looking at bras and said "Hi, can I help at all?’.
The teen in particular felt very uncomfortable. They left, and their mum complained to M&S that this was inappropriate.
Having worked there I agree, and don't recall any biological men approaching women for help with bras. Many will be uncomfortable.
Unfortunately many on TRA have claimed that there's no evidence this person was "trans" and that they might have just been tall.
This "not transism" is a common tactic to claim it will be i.e. butch lesbians who will be disadvantaged by any spaces or practices based on biological sex.