This doesn't seem entirely correct. It may be limitations imposed by power shell or other components...
But I've been able to issue LE certs with bit sizes between 2048 and 4096.
Here is one such example: Auto renewing via cron job only issuing 2048 key size - #6 by rg305
Given very few people will ever even think about getting a cert in between them - this is really a non-issue.
But I just like all facts to be accurate and correct.