This might be an overzealos customer service agent misreading their policy.
Usually/the general policy that covers all European airlines is that you can buy a seat for an under 2, but they must be restrained when the seatbelt sign is on. There are two options for restraining them: The supplementary lap belt on an adult's lap, or a suitable car seat strapped into the booked seat with the plane seatbelt.
What "suitable" means can depend on the airline policy, but if you turn up with a car seat and they deem it unsuitable, they just won't let you use it - it can't be left on the seat unsecured in case of turbulence (heavy potential projectile), so they'll try it in the overhead locker but if it doesn't fit, they will take it into the hold instead (without charging you usually).
But restraining the child is a totally separate thing to actually booking the seat so you can have use of it/extra space. I think sometimes they just say you need to bring a carseat if you book the extra seat to save arguments when somebody tries to say well I've booked this for my baby, why do they need to be on my lap?
In terms of what car seats you can use - there is no seat currently that meets the TUV approval for airlines, nor is approved for use in a car fitted with a lap belt, and is also rear facing in a car. You just don't get lap belt approved seats these days. The car regulations have tightened up and they move around too much in a crash so they aren't made.
The three options that have the TUV approval are:
Nachfolger Hy5 - but this is also rear facing in the plane and a lot of airlines don't like rear facing seats in the plane. Also no use for an everyday seat, good for travel, but very expensive.
A load of infant carriers (popular ones) - but again only rear facing and some airlines don't allow that. Also only usable until the child is approx 80cm tall, which is somewhere between 1-2 years old. If your little one is not incredibly tall, you might get away with a Joie Juva or Britax Babysafe (original) or Maxi Cosi Cabriofix (not i-size). Those ones last a little bit longer for height than standard infant carriers. If you can try one of these out in a shop before you buy, the fact your LO is 18 months might actually make this a feasible option.
Britax Eclipse- but it seems to be discontinued and also it's only forward facing in a car.
What you do sometimes find is that the airline isn't actually that fussy. They tend to look for the R44 approval sticker on the car seat, not necessarily the TUV approval any more (even R44 is a bit old now but they do still exist at least) and they don't always check in the manual whether it's actually approved to fit with a lap belt, as long as it can be belt fitted (not an isofix only seat) and can be forward facing and the bottom part of the seat fits within the airline seat. In terms of extended rear facing seats which do this, I've completely anecdotally heard that Britax Two Way Elite, Joie Tilt (aka Graco Extend) and Joie Steadi all do. I've seen a photo of the TWE and the Joie Tilt on the plane - not my pictures, so can't share and can't link to the source, but you can find them if you go digging on the car seat FB groups.
Just if you do decide to do that, do it in the knowledge that the airline staff could at the last minute decide that your seat isn't officially suitable and needs to go in the hold.