YANBU. You planned ahead and booked the chairs because you do still have a need for them.
Our LO is two years and two months old and we still prefer to use a highchair where possible, mainly because of the height of tables etc.
I think anyone with a child who has ever eaten a meal out has balanced a child on their knee due to a shortage of clean, safe, unbroken highchairs at least once.
Getting a high chair in a restaurant can be a bit of a lottery and we've had several meals spent passing LO between us so we can take turns to feed him and eat ourselves, using his pushchair as a low chair or cutting up our own food into tiny bite size bits so we can balance him, feed him and feed ourselves all one handed.
It's not ideal to have a one and a half year old on your lap while you eat but there was more than one adult in that group to help with the holding and feeding, the woman sounded rude, they could have planned ahead as easily as you did and two and a half is still young enough to use a highchair. It's not ideal to make someone elses two and a half year old kneel on a grown up chair either.
We have a few favourite restaurants locally, one has contained booths that have high seats and low tables, ideal for LO and so no need for a highchair. Another asks if parents can either book a highchair or wait until one is available at busy times and we've had no issue there either as they are happy to give LO something to occupy himself with (carrot sticks, crayons, a free drink etc) while we wait.
They had a few choices open to them, to book ahead, to wait for a highchair to become available or to cope between them with the child on their laps. Politely asking someone else to give up a highchair they were already using might have been an option if they really felt they had to but rudely demanding one should never have crossed their minds.