At face value, yes that’s completely astounding but in the interest of balance there’s more then one type of membership. I’m not a member of the party, but am a member of an affiliated union and pay into the political fund and I get a vote on leadership contests etc same as a regular member so she might have only been a member for 4 years but could have been a member of an affiliated body for much longer. Also might have been a member of the Co-Operative Party? No idea if she is or isn’t, but just putting it out there as possible explanations.
No idea about the Cooperative party membership. I imagine if she had been a member we'd know about it!
I take your point about trade union membership. She was sponsored by Unite, but was she ever a member of the union? She was a commercial solicitor I believe. It's unlikely that the professional association she belonged to is, or was, affiliated to the Labour party.
But....I do insist on individual membership as a token of commitment anyway. There used to a rule in the party - a good one too because it stopped careerists - that you had to be an individual member of the party for 2 years before being eligible to stand as a councillor or MP.
Starmer and Jess Phillips were members of the party when they were 16 and 14 respectively. Long Bailey joined in her 30s in order to become an MP. She was also McCluskey's creature from the start.