By brilliant you apparently mean completely incorrect from start to finish.
Perhaps not quite from start to finish, there are indeed probably people who email at 4.55 and complain that they didn't get a response Monday morning at 9.05, but that is not a relevant comparison and had nothing in common with this situation.
Literally everyone - and I do mean absolutely everyone who has read this thread - recognises that people can be busy all day at work, and most of us have also been in that position. It's not, in any way, a surprising revelation.
For privacy reasons, I am not allowed my phone at work, it's in a locker which I can access on breaks.
So when I am on one of my breaks (which everyone including the OP gets no matter what their job) I check my phone and if there are important messages I respond taking a few seconds to say I am at work and will check in later.
Just like the OP I, and everybody else who has read this thread, prioritise who I respond to based on my own needs and abilities at that time.
The OP did not prioritise the text and decided not to take a few seconds to say "Sounds great, will contact you after work". And that's fine.
The other person then moved on, so that is also fine.
So actually, this post is pointless, and the reply you quoted was incorrect and based on faulty assumptions.