2RX4 vs 2RX8 DIMM Question. Vendor put different DIMM type than the Manual specifies for the config

I just bought a used R730 LFF from a reseller.  It is replacing an R710 for light use in a very small office environment.  It will be an ESXi host for about five VM's:  vCenter, a Shoretel phone server (two users, three trunks), and a few other miscellaneous VM's.

The machine has dual E5-2680v4 CPUs.  I updated BIOS and firmware to the latest.

The vendor shipped it with 96GB of 2RX4 DDR4 2400 DIMMs.  The manual does not list that as a supported memory configuration for two reasons.

(For 96GB, the manual does not list 16GB DIMMs with dual CPUs--it lists either 4GB or 8GB DIMMs.  The manual does list 16GB DIMMs for a 128GB configuration.  I will buy two extra DIMMs to remedy that and use all four memory channels.)

Here is the pending question:  For dual CPUs with 128GB, the manual specifies only 2RX8 for 16GB DIMMs. In fact, the manual specifies 2RX8 for every memory configuration using 16GB DIMMs.

I raised this with the vendor and he says that it makes no difference and that they configure severs all the time using either 2RX4 or 2RX8.  (Interchangeably, using one or the other for the same model Dell servers.)

I can only go by what the manual says but the vendor dismissed it out of hand.  "We do it all the time" is not very convincing to me, particularly since the manual asserts otherwise.

What effect will there be from using 2RX4 16GB DIMMs in this machine?  I imagine Dell had a reason to specify 2RX8 with this config. I've searched a lot of articles and posts and not found anything on point.  I found plenty of threads about not mixing types.  Others about whether dual or single rank are specified.  A few threads pointed to dead links.

Can anyone elaborate? Would it have an effect on the performance of the RAM?  If there is any downside to using 2RX4 in this config, I will insist on the 2RX8.

The vendor was getting annoyed that I brought it up.  In the end, it sounded like he would swap the RAM.  Given his hard pushback, that's not guaranteed and I'd like to have as much info as I can.

(A secondary reason for wanting the RAM swapped is that I wanted it to match the R730xd machine being used for backups (and as backup host).  It has 16GB 2RX8 DIMMs.  I had asked the vendor to make sure it was the same RAM and he whiffed.)