> Should I treat this as some datapoint on monero’s security having held up well so far?

No. The reason attackers mine Monero and not some other cryptocurrency isn't the anonymity. Most cryptocurrencies aren't (meaningfully) mineable with CPUs, Monero (apparently) is. There may be others, but I suspect that they either deliver less $ per CPU-second (due to not being valuable), or are sufficiently unknown and/or painful to set up that the attackers just go with the known default.

Trying to mine Bitcoin directly would be pointless, you'd get no money because you're competing with ASIC miners. Some coins were designed to be ASIC resistant, these are mostly mined on GPUs. Monero (and some other coins) were designed to also be GPU resistant (I think). You could see it as a sign that that property has held up (well enough), but nothing else.