It sounds like they are probably referring to the processing power of your firewall, rather than any specific compatibility.
Your new APs are very likely capable of carrying significantly more traffic than any they are replacing. How much of that extra capacity your wifi client base can realistically make use of will depend on your device mix and how they are used (what apps etc). You have to think that those devices will become more capable, over the years though, as they're replaced.
Moreover, you may well be (more) constrained by your available Internet uplink, beyond your firewall. If your link is just 100 Mbps or so, how will a Gig-capable firewall help? That having been said, your Internet pipe will probably be upgraded, within the lifespan of your APs and a replacement firewall anyway. I try to think about your whole traffi-flows and any bottlenecks along the way.
Bear in mind too that you needn't necessarily upgrade the existing Fortigate with a new Fortigate; what about an MX? 😏