It takes the potential total electorate in the leadership contest to 610,753.
Took me aabout 15 seconds to find that on Google.
I fully accept that 600000 people did not join the party then but there were over 600000 members.

It doesn't even make a claim for 600,000+ members (rather than the claim of 600,000 new members) as it goes on to say (about the figure of 610,753) that: Labour says checks are still taking place on all new members to weed out non-party supporters. That is presumably why they quote the figure as 'potential'.

As to it being pointless correcting Steve. If the original statement is allowed to stand then others can claim that they read it somewhere, and claim it as fact. Which is what Steve also appears to have been doing (mistakenly).