the BTC stack in orange always keeps going up, making it a personal ATH for him every time.
That's not what an All Time High means. It's DCA (Dollar Cost Averaging): buying a fixed amount into an investment on regular moments.
Yeah, ATH is basically the highest fiat value of Bitcoin at all times.
I'm just mentioning here the fact that this person never has sold BTC, which means that every time he buys 500 euros worth of Bitcoin to add to their stack, their personal maximum amount of Bitcoin they have ever held continues to increase into a new maximum.
It's simply using the phrase All Time High in another context, like a personal best.