MI100 is hardly the first AMD datacenter GPU. The first Instinct branded card, MI25, is from 2017 [1]. But ATI/AMD had FirePro/FireStream branded GPGPU cards going back to the mid 2000s [2,3]. They just never caught on because AMD's software, support and marketing was not competitive with Nvidia's.
MI25 is a Vega 10 (same as Vega 56/64) and the MI50 is Vega 20 (same as Radeon VII). CDNA is just a Vega variant too, you'll see that MI25/Vega 56/Vega 64 are all gfx9* generation hardware just like CDNA1 and CDNA2 [1], while later RDNA cards are gfx10* and gfx11*.
But, what difference does it make? Nvidia also shipped the same _architecture_ for their datacenter and consumer cards for quite a few generations back then (e.g. Pascal), though typically not the same die. Whether they reuse the same architecture or not, they had a product that they marketed as enterprise/datacenter cards. The buyers don't care if it's a rebranded consumer card or not as long as it works well - see the Nvidia L40S (uses AD102 - same as RTX4090 [2]) which is very popular in inference.
Not to mention, with GCN, AMD made an explicit bet on unifying their architecture for compute & graphics. They bet on being able to supply both the consumer and datacenter markets with the same silicon by coming up with graphics hardware that was quite compute-heavy (hence why AMD consumer cards were stronger against their Nvidia counterparts until the Ampere generation or so).
I don't think having a common ancestry for the ISA means much, or even having the same ISA.
Anyway, I don't understand what you want from me or are arguing about.
They were trying to win the datacenter CPU market and not the GPU market. They did well at that. They've recently started trying to win the GPU market as well, cause now they can afford to. They seem to be doing well now.
I'm saying that the "not trying to win the datacenter GPU market" bit is not quite correct, as they had a lot of products trying to address that market. Agree that their offerings today are a marked improvement over the previous ones though.
[1] https://www.techpowerup.com/gpu-specs/radeon-instinct-mi25.c...
[2] https://en.wikipedia.org/wiki/AMD_FireStream
[3] https://en.wikipedia.org/wiki/AMD_FirePro