That's true. The balance between "incoming" and "outgoing" heat is still
negative for some weeks after the shortest day. Similarly, in summer on
average July and August are warmer than June.

And even in September the temperature is often still between 15 and 20C
in the late evening, with people eating outdoors...

That said, the strong sun in June makes a difference as to how it
feels, along with the fact we're not used to heatwaves yet. I remember
on June 11th last year being out in a day of 19C and strong sunshine.
It felt hot. Same temperatures at the end of August and it felt
autumnally cold...

Nick