Is the FPGA Smatfusion2 claimed to be highly secure by its manufacturer (Microsemi, now Mircochip) because it is a non volatile memory FPGA? Meanwhile the SRAM FPGA like the zynq 7000 are considered vulnerable because the content need to be loaded each time power is applied to the system? Is that enough to ensure secure booting?
Nothing is absolutely secure. There are degrees of security. While higher security is often advantageous, there are usually tradeoffs to get it. These can be higher cost, more difficulty in working with and doing development using the product, etc.
Storing the configuration inside the FPGA does provide higher security than having the configuration external and having to load it into the FPGA at powerup. Whether that's secure enough for your purposes is something only you can say. Meaningless words like "highly" are just that, meaningless.