I think the claim goes back to Galland's recollection of the bombs available to the Luftwaffe in 1940 which must itself have been second hand. The source for many authors' claims about the doubt when the Germans introduced their SD and PC bombs, particularly the heavier ones, is probably "Air Power and the Royal Navy, 1914-1945" by G Till, published in 1979. I don't have a copy and don't know his source.
However the above Luftwaffe document from April 1940 makes clear that the Luftwaffe possessed both large, armor piercing and rocket assisted armor piercing bombs as of April 1940 suitable against ships, particularly the older ones. So G Till seems to be in error.
It's a bit of a moot point since even if the B-2 could in fact carry a 1000Kg weapon (contrary to my loading plan, but according to many reputable sources) it certainly couldn't carry anything larger. I'd have to check, but I don't think that the Jumo 211 D engined B-2 equipped most Luftwaffe Ju 87 units in September 1940 in any case . I can't find any pictures, after a quick look, of a B series JU 87 carrying anything bigger than a 500Kg bomb either, but I'll happily be corrected. The limited endurance of the Ju 87 (about 300Km) before the introduction of the R is also often overlooked by proponents of Sea Lion.
The Ju 87R was present in Norway IIRC or introduced at around the same time. The standard Ju 87B could carry an 1000 kger, I tend to believe it was more of a question of adding the suitable bombing rack, but the problem is moot since the rocket assisted PC 500 RS series were developed just to address the problem of anti shipping strikes from low altitude.
There was also the Ju 88 of course, which could carry large bombs and far away.
The first PC1400 bombs reported by the British were found in the Bristol area in 1942. The British surmised that the larger bombs were developed due to the inability of the 1000Kg version(s) to sink a large ship like HMS Illustrious despite several hits.
Given that the British usually found unexploded versions of new bombs or mines, and ordnance fitted with new fuses or timers within days or weeks (often days) of their deployment I doubt that they were in use much earlier. Certainly not in 1940.
Certainly this logic is very flawed. what british intel speculation about german equipment (which was very often bordering phantasmagoria, see "fake smoke" devices speculated on aircraft) is quite irrelevant as to what the Germans actually had. The larger bombs, especially the AP ones were meant against specially hard targets, which did not turn up either at sea in 1940 (for a destroyer sized target, the standard high capacity 250 kgers were more optimal). I doubt the British fished out many unexploded German bombs from the Channel or from the Dunkerque beaches... they had the ability to inspect a number of unexploded ones dropped on the mainland, but those represented the ones believed to be optimal against prime mainland targets (airfields, light housing and brick buildings, shops, factories, docks, ie. overwhelmingly 50 and 250 kgers were to be used against building with less than three levels, ie. practically all british housing). IOW, why would the Luftwaffe, in 1940, drop one-and-a-half ton
armor piercing bombs on airfields, brick houses or docks...?
Obviously there was very little chance that the British would be aware of the larger bombs early.