Beyond all that, a PowerShell cmdlet could be implemented as a .net class, a script, or a function with cmdlet style binding, or literally anything else you can write a command processor for. (I worked on the team that developed PowerShell for about 8 years, so don't argue with me, n00b.)
"We do not know if the paid-for version of DirectMailer also include the backdoor or not."
I wonder if the paid-for version actually exists or if the whole site is just a setup for spreading the malware, by getting people to download the supposedly cracked version. I can't imagine a lot of folks are spending $240 to download spam software from some shady website.