I'm gob-smacked that you're gob-smacked by this!
Just managing your toil and not having to account for it has been the case in every place I've worked since my first full time job in 2009 - only I actually can manage to open my mind and understand that other places might do things differently.
"Otherwise everyone could rack up 15 minutes a night by stopping for a chat, washing their coffee mug and retrieving leftover lunch from the fridge!"
Well, yeah, but they could also 'waste' 15 mins at any point in the day doing this if they didn't have TOIL. I'm going to completely blow your mind and tell you that both in my current job and my previous one we could accrue toil even when WFH and travelling for work - so we could say we were working for 30 mins when we were actually cooking tea or watching TV, or saying a journey took 3 hrs because of traffic when it actually only took 2.5, and nobody would know, check or care.
That's why it's for the organisation to put in processes to limit potential piss-taking - as other posters have said, being able to accrue a whole week is very unusual, there are usually limitations to the amount you can accrue before carrying it over.
But the whole point of TOIL/flexitime is that they are benefits - the idea is that the organisation trusts staff to use it responsibly, and for the few that don't it is for their manager to identify and... you know, manage.
It's really not hard to see when the odd person is taking the piss. Managers don't have to scrutinise timesheets. If someone like OP's colleague adds an extra hour every day, and ends up with 20 hours toil that month, it should be obvious for the manager to say 'well your output should be approximately 15% higher than usual, if it isn't, where did the time come from? What were you doing?'
However for the majority of people it doesn't work like that. They have a two hour lunch break to go to an appointment then stay an extra hour late = overall the same amount of hours and the same amount of work done.
Or they stay after work to help with a conference one evening - only takes a second for manager to look over the time sheet and see 'X accrued 4 hours toil on one day - oh yeah that was when she stayed late to work for the conference so worked 12 hrs rather than 8.' Or 'I see everyone on my team accrued 1-2 hours toil a day for the last 2 weeks at the end of march when everyone worked like mad to finalise the year end figures, completely expected.'