High chair - what age with supporting cusion?

[Anticyclone]

I know the advice is for you to not put baby in a high chair before they can sit up unsupported.

But does a supporting cushion like this:
Ikea link
mean you can put them in earlier?

Our (not unsupported sitting yet) almost 4 month old is getting annoyed with his bouncer and much prefers to be up at the table on someone's knee. I was wondering if this might be a way for him to sit at the table with us while leaving us both hands to eat with!

I don't know of any specific advice but I used a rolled up towel for mine to pad the gaps until he looked like he didn't need it. I planned to get the Ikea one but they didn't have any the day I went so I improvised and it seemed to work ok. I think he did enjoy a change of scenery and I put toys on the tray for him.